chore: upgrade Flutter to 3.38.7 (#37)

- Automated Flutter upgrade from 3.38.1 to 3.38.7
- Applied automatic fixes with dart fix
- Updated tracked Flutter version
- Verified with flutter analyze and flutter test

Co-authored-by: GitHub Action <action@github.com>

Author: github-actions[bot]

.flutter-version-tracked

@@ -1 +1 @@
-3.38.1
+3.38.7