[flutter_template_images] Add macOS app icon assets (#1930)

Adds the the AppIcon assets in the macOS app template while
simultaneously updating them to meet Apple's icon guidelines as of macOS
Big Sur.

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

Issue: https://github.com/flutter/flutter/issues/103371

packages/flutter_template_images/CHANGELOG.md

@@ -1,6 +1,7 @@
-## NEXT
+## 4.1.0
 
 * Updates package description.
+* Adds macOS template app icons, updated to Big Sur style.
 
 ## 4.0.0
 

packages/flutter_template_images/pubspec.yaml

@@ -2,7 +2,7 @@ name: flutter_template_images
 description: Image files for use in flutter_tools templates without adding binary files to flutter/flutter.
 repository: https://github.com/flutter/packages/tree/main/packages/flutter_template_images
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+flutter_template_images%22
-version: 4.0.0
+version: 4.1.0
 
 environment:
   sdk: ">=2.1.0 <3.0.0"