Always use new plugin schema for plugin template (#48258)

Now that the new schema is supported on the stable channel, and the old
schema is considered legacy, the template should always create plugins
using the new schema.

packages/flutter_tools/lib/src/commands/create.dart

@@ -642,10 +642,6 @@ To edit platform code in an IDE see https://flutter.dev/developing-packages/#edi
       'web': web,
       'macos': macos,
       'year': DateTime.now().year,
-      // For now, the new plugin schema is only used when a desktop plugin is
-      // enabled. Once the new schema is supported on stable, this should be
-      // removed, and the new schema should always be used.
-      'useNewPluginSchema': macos,
       // If a desktop platform is included, add a workaround for #31366.
       // When Linux and Windows are added, we will need this workaround again.
       'includeTargetPlatformWorkaround': false,

packages/flutter_tools/templates/plugin/pubspec.yaml.tmpl

@@ -21,23 +21,9 @@ dev_dependencies:
 # The following section is specific to Flutter.
 flutter:
   # This section identifies this Flutter project as a plugin project.
-{{^useNewPluginSchema}}
-  # The androidPackage and pluginClass identifiers should not ordinarily
-  # be modified. They are used by the tooling to maintain consistency when
-  # adding or updating assets for this project.
-  plugin:
-    androidPackage: {{androidIdentifier}}
-    pluginClass: {{pluginClass}}
-{{/useNewPluginSchema}}
-{{#useNewPluginSchema}}
   # The 'pluginClass' and Android 'package' identifiers should not ordinarily
   # be modified. They are used by the tooling to maintain consistency when
   # adding or updating assets for this project.
-  #
-  # NOTE: This new plugin description format is not supported on Flutter's
-  # stable channel as of 1.9.1. A plugin published using this format will not
-  # work for most clients until the next major stable release.
-  # However, it is required in order to declare macOS support.
   plugin:
     platforms:
       android:
@@ -45,9 +31,10 @@ flutter:
         pluginClass: {{pluginClass}}
       ios:
         pluginClass: {{pluginClass}}
+{{#macos}}
       macos:
         pluginClass: {{pluginClass}}
-{{/useNewPluginSchema}}
+{{/macos}}
 
   # To add assets to your plugin package, add an assets section, like this:
   # assets: