Update year2023 flag deprecation message (#162607)

Fixes [Improve `year2023` flag deprecation
message](https://github.com/flutter/flutter/issues/162606)

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- [x] I updated/added relevant documentation (doc comments with `///`).
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packages/flutter/lib/src/material/progress_indicator.dart

@@ -343,7 +343,8 @@ class LinearProgressIndicator extends ProgressIndicator {
     this.stopIndicatorRadius,
     this.trackGap,
     @Deprecated(
-      'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+      'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
       'This feature was deprecated after v3.26.0-0.1.pre.',
     )
     this.year2023,
@@ -420,7 +421,8 @@ class LinearProgressIndicator extends ProgressIndicator {
   ///
   /// If [ThemeData.useMaterial3] is false, then this property is ignored.
   @Deprecated(
-    'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+    'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+    'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
     'This feature was deprecated after v3.27.0-0.1.pre.',
   )
   final bool? year2023;
@@ -719,7 +721,8 @@ class CircularProgressIndicator extends ProgressIndicator {
     this.constraints,
     this.trackGap,
     @Deprecated(
-      'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+      'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
       'This feature was deprecated after v3.27.0-0.1.pre.',
     )
     this.year2023,
@@ -749,7 +752,8 @@ class CircularProgressIndicator extends ProgressIndicator {
     this.constraints,
     this.trackGap,
     @Deprecated(
-      'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+      'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
       'This feature was deprecated after v3.27.0-0.2.pre.',
     )
     this.year2023,
@@ -832,7 +836,8 @@ class CircularProgressIndicator extends ProgressIndicator {
   ///
   /// If [ThemeData.useMaterial3] is false, then this property is ignored.
   @Deprecated(
-    'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+    'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+    'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
     'This feature was deprecated after v3.27.0-0.2.pre.',
   )
   final bool? year2023;

packages/flutter/lib/src/material/progress_indicator_theme.dart

@@ -49,7 +49,8 @@ class ProgressIndicatorThemeData with Diagnosticable {
     this.trackGap,
     this.circularTrackPadding,
     @Deprecated(
-      'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+      'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
       'This feature was deprecated after v3.27.0-0.2.pre.',
     )
     this.year2023,
@@ -131,7 +132,8 @@ class ProgressIndicatorThemeData with Diagnosticable {
   ///
   /// If [ThemeData.useMaterial3] is false, then this property is ignored.
   @Deprecated(
-    'Use ProgressIndicatorTheme to customize the ProgressIndicator appearance. '
+    'Set this flag to false to opt into the 2024 progress indicator appearance. Defaults to true. '
+    'In the future, this flag will default to false. Use ProgressIndicatorThemeData to customize individual properties. '
     'This feature was deprecated after v3.27.0-0.2.pre.',
   )
   final bool? year2023;

packages/flutter/lib/src/material/slider.dart

@@ -239,7 +239,8 @@ class Slider extends StatefulWidget {
     this.autofocus = false,
     this.allowedInteraction,
     @Deprecated(
-      'Use SliderTheme to customize the Slider appearance. '
+      'Set this flag to false to opt into the 2024 slider appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use SliderThemeData to customize individual properties. '
       'This feature was deprecated after v3.27.0-0.1.pre.',
     )
     this.year2023,
@@ -573,7 +574,8 @@ class Slider extends StatefulWidget {
   ///
   /// If [ThemeData.useMaterial3] is false, then this property is ignored.
   @Deprecated(
-    'Use SliderTheme to customize the Slider appearance. '
+    'Set this flag to false to opt into the 2024 slider appearance. Defaults to true. '
+    'In the future, this flag will default to false. Use SliderThemeData to customize individual properties. '
     'This feature was deprecated after v3.27.0-0.1.pre.',
   )
   final bool? year2023;

packages/flutter/lib/src/material/slider_theme.dart

@@ -298,7 +298,8 @@ class SliderThemeData with Diagnosticable {
     this.thumbSize,
     this.trackGap,
     @Deprecated(
-      'Use SliderTheme to customize the Slider appearance. '
+      'Set this flag to false to opt into the 2024 slider appearance. Defaults to true. '
+      'In the future, this flag will default to false. Use SliderThemeData to customize individual properties. '
       'This feature was deprecated after v3.27.0-0.2.pre.',
     )
     this.year2023,
@@ -637,7 +638,8 @@ class SliderThemeData with Diagnosticable {
   ///
   /// If [ThemeData.useMaterial3] is false, then this property is ignored.
   @Deprecated(
-    'Use SliderTheme to customize the Slider appearance. '
+    'Set this flag to false to opt into the 2024 slider appearance. Defaults to true. '
+    'In the future, this flag will default to false. Use SliderThemeData to customize individual properties. '
     'This feature was deprecated after v3.27.0-0.2.pre.',
   )
   final bool? year2023;