[macOS] Update template icons to Big Sur style (#103739)

Updates the AppIcon assets in the macOS app template to meet Apple's
icon guidelines as of macOS Big Sur. Assets were moved to the
[flutter_template_images][1] package, which includes them as of version
4.1.0.

Ref: https://developer.apple.com/design/human-interface-guidelines/macos/icons-and-images/app-icon/

Issue: flutter/flutter#103371

[1]: https://github.com/flutter/packages/tree/main/packages/flutter_template_images

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

@@ -30,7 +30,7 @@ const Map<String, String> kManuallyPinnedDependencies = <String, String>{
   // Add pinned packages here. Please leave a comment explaining why.
   'archive': '3.1.11', // Breaking changes in 3.2.0, see https://github.com/flutter/flutter/issues/98536
   'flutter_gallery_assets': '1.0.2', // Tests depend on the exact version.
-  'flutter_template_images': '4.0.0', // Must always exactly match flutter_tools template.
+  'flutter_template_images': '4.1.0', // Must always exactly match flutter_tools template.
   // "shelf" is pinned to avoid the performance regression from a reverted
   // feature from https://github.com/dart-lang/shelf/issues/189 . This can be
   // removed when a new major version of shelf is published.

packages/flutter_tools/pubspec.yaml

@@ -16,7 +16,7 @@ dependencies:
   coverage: 1.3.1
   crypto: 3.0.2
   file: 6.1.2
-  flutter_template_images: 4.0.0
+  flutter_template_images: 4.1.0
   html: 0.15.0
   http: 0.13.4
   intl: 0.17.0
@@ -104,4 +104,4 @@ dartdoc:
   # Exclude this package from the hosted API docs.
   nodoc: true
 
-# PUBSPEC CHECKSUM: f082
+# PUBSPEC CHECKSUM: 8f83

packages/flutter_tools/templates/template_manifest.json

@@ -1,6 +1,6 @@
 {
     "version": 1.0,
-    "_comment": "A listing of all possible template output files.",
+    "_comment": "A listing of all possible template output files. Files ending in .img.tmpl correspond to files checked in to the flutter_template_images package in the flutter/packages repo, located at the same path, excluding the .img.tmpl suffix.",
     "files": [
         "templates/app/lib/main.dart.tmpl",
         "templates/app/pubspec.yaml.tmpl",
@@ -98,13 +98,13 @@
         "templates/app_shared/macos.tmpl/Runner.xcworkspace/contents.xcworkspacedata",
         "templates/app_shared/macos.tmpl/Runner.xcworkspace/xcshareddata/IDEWorkspaceChecks.plist",
         "templates/app_shared/macos.tmpl/Runner/AppDelegate.swift",
-        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_1024.png",
-        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_128.png",
-        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_16.png",
-        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_256.png",
-        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_32.png",
-        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_512.png",
-        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_64.png",
+        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_1024.png.img.tmpl",
+        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_128.png.img.tmpl",
+        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_16.png.img.tmpl",
+        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_256.png.img.tmpl",
+        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_32.png.img.tmpl",
+        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_512.png.img.tmpl",
+        "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/app_icon_64.png.img.tmpl",
         "templates/app_shared/macos.tmpl/Runner/Assets.xcassets/AppIcon.appiconset/Contents.json",
         "templates/app_shared/macos.tmpl/Runner/Base.lproj/MainMenu.xib",
         "templates/app_shared/macos.tmpl/Runner/Configs/AppInfo.xcconfig.tmpl",