Revert "sync: dropped "NOTICE: --use-superproject is in beta ..." message."

This reverts commit d53cb9549a. As long as
repo's reference docs treat this feature as a work in progress and don't
cover it well enough to allow all repo maintainers to easily support it,
it is inconsistent to report to users that it is no longer in beta.
Thanks for vapier@google.com for noticing.

https://crbug.com/gerrit/15527 tracks the required documentation changes
before we'd be ready to roll forward again.

Change-Id: Ic9bd951cfb3c1abf6e1bfa30dfe4afa1c9b7bec6
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/327337
Reviewed-by: Jonathan Nieder <jrn@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Jonathan Nieder <jrn@google.com>

git_superproject.py

@@ -226,6 +226,8 @@ class Superproject(object):
                        f'{self._manifest.manifestFile}')
       return SyncResult(False, False)
 
+    print('NOTICE: --use-superproject is in beta; report any issues to the '
+          'address described in `repo version`', file=sys.stderr)
     should_exit = True
     if not self._remote_url:
       self._LogWarning(f'superproject URL is not defined in manifest: '