Reenable linux_web_engine mac tests on Mac-14 (#162409)

Mac-14 bots are on macOS 14.7 (previously 14.5), which [presumably has
Safari
18](https://github.com/flutter/flutter/issues/150023#issuecomment-2479664389)
and may fix the errors we were seeing in
https://github.com/flutter/flutter/issues/150023.

Let's run them on Mac-14 again and see how they do.

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engine/src/flutter/ci/builders/linux_web_engine_test.json

@@ -519,7 +519,7 @@
       "recipe": "engine_v2/tester_engine",
       "drone_dimensions": [
         "device_type=none",
-        "os=Mac-13",
+        "os=Mac-13|Mac-14",
         "cpu=arm64"
       ],
       "gclient_variables": {

engine/src/flutter/lib/web_ui/dev/generate_builder_json.dart

@@ -100,7 +100,7 @@ class GenerateBuilderJsonCommand extends Command<bool> {
         packageLock,
         'Mac',
         BrowserName.safari,
-        specificOS: 'Mac-13',
+        specificOS: 'Mac-13|Mac-14',
         cpu: 'arm64',
       ),
     ];