repo-updater: Implement GitHub repo enabled state deprecation migration

Created by: tsenart

This change-set implements the migration that deprecates the enabled state of GitHub repos that'll be under management of the new syncer. This is done to ensure that previously disabled repos are excluded from syncing and repos that would be deleted but are enabled keep being synced.

The UI changes that remove the enable / disable feature will follow up.

Part of #2025 (closed)