Reintroduce BBM to migrate ci_builds_metadata for self-managed

In #552069 (closed) we added a background migration to move data into the new destinations, but we limited it to execute only on .com to ensure that it works as expected. We should reintroduce the migration for self-managed too.

Proposal

  1. create a new post deployment migration similar to the one in the .com migration but it returns early for .com
  2. Create a new background migration that inherits from the original one and used that in the pdm
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