Geo: Deprecate Git repo redownload logic

Problem

Geo's Redownload logic does not align with Gitaly and Gitaly Cluster. The Redownload logic breaks encapsulation, and assumes repositories can be moved around easily and safely. We could do a lot of work to implement this in Gitaly. Or we could do the following proposal. See &8208 for more information.

Proposal

Stop using Redownload logic.

• Add deprecation release post in 15.9 for removing the Redownload button from /admin/geo/replication/projects. In order to address data consistency issues with the redownload approach.
• In 16.0, add feature flag :geo_deprecate_redownload
  • Use the FF in repository_base_sync_service.rb and framework_repository_sync_service.rb to block redownload when enabled. Default it enabled in 16.0. => !118411 (merged)
• Open follow up issue to remove redownload code and the FF in 16.1+ => #408902 (closed)