Allow for specifying only MAJOR.MINOR Flutter version (without the "x" suffix)

[bartekpacia]

Allow for specifying Flutter version like this:

- name: Set up Flutter
  uses: subosito/flutter-action@v2
  with:
    channel: stable
    flutter-version: 3.19

in addition to the aleady supported:

- name: Set up Flutter
  uses: subosito/flutter-action@v2
  with:
    channel: stable
    flutter-version: 3.19.x

This would make us more consistent with the official dart-lang/setup-dart action.