build(deps): bump subosito/flutter-action from 2.12.0 to 2.15.0

Bumps [subosito/flutter-action](https://github.com/subosito/flutter-action) from 2.12.0 to 2.15.0.
- [Release notes](https://github.com/subosito/flutter-action/releases)
- [Commits](subosito/flutter-action@v2.12.0...v2.15.0)

---
updated-dependencies:
- dependency-name: subosito/flutter-action
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/build_all.yml

@@ -14,7 +14,7 @@ jobs:
         version: [ stable ]
     steps:
       - uses: actions/checkout@v4
-      - uses: subosito/flutter-action@v2.12.0
+      - uses: subosito/flutter-action@v2.15.0
         with:
           channel: ${{ matrix.version }}
       - name: Build & format for flutter_kiwi example

.github/workflows/dart_kiwi_example.yml

@@ -14,7 +14,7 @@ jobs:
         working-directory: examples/dart_kiwi
     steps:
       - uses: actions/checkout@v4
-      - uses: subosito/flutter-action@v2.12.0
+      - uses: subosito/flutter-action@v2.15.0
         with:
           channel: 'stable'
       - run: flutter packages get

.github/workflows/flutter_kiwi_example.yml

@@ -14,7 +14,7 @@ jobs:
         working-directory: examples/flutter_kiwi
     steps:
       - uses: actions/checkout@v4
-      - uses: subosito/flutter-action@v2.12.0
+      - uses: subosito/flutter-action@v2.15.0
         with:
           channel: 'stable'
       - run: flutter packages get