Fix IconButton leaks its internal MaterialStatesController (#130720)

## Description

This PR adds a call to dispose the internal `MaterialStatesController` instantiated by `_SelectableIconButtonState`.

I found this memory leak while working on M2/M3 test update for `about_test.dart`. This memory leak only happens when using M3 because `IconButton` relies on `_SelectableIconButton` only when useMaterial3 is true:

3a1190a5a8/packages/flutter/lib/src/material/icon_button.dart (L671-L721)

## Related Issue

Fixes https://github.com/flutter/flutter/issues/130708

## Tests

Adds 1 test.

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

@@ -880,6 +880,12 @@ class _SelectableIconButtonState extends State<_SelectableIconButton> {
       ),
     );
   }
+
+  @override
+  void dispose() {
+    statesController.dispose();
+    super.dispose();
+  }
 }
 
 class _IconButtonM3 extends ButtonStyleButton {

packages/flutter/test/material/icon_button_test.dart

@@ -8,6 +8,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 
+import '../foundation/leak_tracking.dart';
 import '../rendering/mock_canvas.dart';
 import '../widgets/semantics_tester.dart';
 import 'feedback_tester.dart';
@@ -2770,6 +2771,25 @@ void main() {
 
       expect(tester.takeException(), isNull);
     });
+
+    testWidgetsWithLeakTracking('Material3 - IconButton memory leak', (WidgetTester tester) async {
+      // This is a regression test for https://github.com/flutter/flutter/issues/130708.
+      Widget buildWidget(bool showIconButton) {
+        return showIconButton
+          ? MaterialApp(
+              theme: ThemeData(useMaterial3: true),
+              home: IconButton(
+                onPressed: () { },
+                icon: const Icon(Icons.search),
+              ),
+            )
+          : const SizedBox();
+      }
+      await tester.pumpWidget(buildWidget(true));
+      await tester.pumpWidget(buildWidget(false));
+
+      // No exception is thrown.
+    });
   });
 }