When #101960 is done, Flutter apps will be migrated to target minimum iOS 11 instead of 9.

Match the same in the new plugin template. Up the Flutter SDK constraint to whatever version #101960 ships on to enforce that the 11.0 migration has happened on the app side.

Apps on versions of Flutter older than #101960 version will not be able to use these new plugins.

See also the last two times this was done:
#62605
#89695