Part of: #91605

Updated the common buttons: ElevatedButton, OutlinedButton, and TextButton with support for Material Design 3.

In order to use the common buttons with the new Material 3 defaults, turn on the useMaterial3 flag in the ThemeData:

  return MaterialApp(
    theme: ThemeData(useMaterial3: true),
    // ...
  );

In addition to the new look, the AppBar and AppBarTheme now support surfaceTintColor and scrolledUnderElevation properties to support the new elevation visuals needed for Material 3.

By default in Material 3, the app bar will no longer have a drop shadow, but instead will show changes in elevation by applying a surface tint overlay with and opacity based on the elevation on top of the background color. In addition the default elevation is 0 when there is nothing under the app bar. This changes to 3 when something is scrolled beneath the app bar. Example of this behaviour:

Fixes: #92093

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.