Upgrade GitHub Actions to latest versions

[salmanmkc]

Summary

Upgrade GitHub Actions to their latest versions for improved features, bug fixes, and security updates.

Changes

Action	Old Version(s)	New Version	Release	Files
Apple-Actions/import-codesign-certs	v2	v6	Release	build.yml
Apple-Actions/upload-testflight-build	v1	v4	Release	build.yml
apple-actions/import-codesign-certs	v3	v6	Release	build.yml
softprops/action-gh-release	v1	v2	Release	build.yml
subosito/flutter-action	v2.16.0	v2.21.0	Release	build.yml

Why upgrade?

Keeping GitHub Actions up to date ensures:

• Security: Latest security patches and fixes
• Features: Access to new functionality and improvements
• Compatibility: Better support for current GitHub features
• Performance: Optimizations and efficiency improvements

⚠️ Breaking Changes

• apple-actions/import-codesign-certs (v3 → v6): Major version upgrade — review the release notes for breaking changes
• softprops/action-gh-release (v1 → v2): Major version upgrade — review the release notes for breaking changes
• Apple-Actions/import-codesign-certs (v2 → v6): Major version upgrade — review the release notes for breaking changes
• Apple-Actions/upload-testflight-build (v1 → v4): Major version upgrade — review the release notes for breaking changes

Security Note

Actions that were previously pinned to commit SHAs remain pinned to SHAs (updated to the latest release SHA) to maintain the security benefits of immutable references.

Testing

These changes only affect CI/CD workflow configurations and should not impact application functionality. The workflows should be tested by running them on a branch before merging.