Increase plugin template minimum supported macOS version from 10.11 to 10.13, match Flutter SDK constraint #107739
Description
As of #107689 Flutter apps will be migrated to target minimum macOS 10.13.
Match the same in the new plugin template. Up the Flutter SDK constraint to whatever version #107689 ships on to enforce that the 10.13 migration has happened on the app side.
Apps on versions of Flutter older than that version will not be able to use these new plugins.
See also the last two times this was done for iOS:
#62605
#89695
As well as the currently pending #101961