Change the default for`ThemeData.useMaterial3` to true (#129724)

Changes the default for `ThemeData.useMaterial3` to true.

See https://github.com/flutter/flutter/issues/127064

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

@@ -439,7 +439,7 @@ class ThemeData with Diagnosticable {
     pageTransitionsTheme ??= const PageTransitionsTheme();
     scrollbarTheme ??= const ScrollbarThemeData();
     visualDensity ??= VisualDensity.defaultDensityForPlatform(platform);
-    useMaterial3 ??= false;
+    useMaterial3 ??= true;
     final bool useInkSparkle = platform == TargetPlatform.android && !kIsWeb;
     splashFactory ??= useMaterial3
       ? useInkSparkle ? InkSparkle.splashFactory : InkRipple.splashFactory