Test migration mode

Context

This is part of the work to upgrade and migrate the GitLab.com container registry to a new version backed by a metadata database and online garbage collection (&5523 (closed)). This will be achieved following the gradual migration plan detailed in #374 (closed).

Task

Test the migration mode, simulating the production rollout as described in &6442 (closed) while validating:

• Logs and metrics are accurate and sufficient;
• Feature flags behave as expected when toggled on/off for specific actors or have their percentage adjusted;
• Data in storage backend respects the old vs. new root prefix split;
• Data in the metadata database is consistent with storage backend;
• Online GC behaves as expected with two storage prefixes (ignores the old one);
• Overall data integrity.