Allow moving forked projects across shards

What does this MR do?

This allows creating multiple pool repositories for the same project across different shards.

This would occur when a project that was already the source of some forks on a first shard was moved to a new shard.

In this case the old pool repository will continue to exist, containing all the deduplicated objects from the forks on the first shard. But we'll create a new PoolRepository for the shart the source project was moved to.

Then, new forks will be deduplicated using the new PoolRepository on the new shard.

Ref: #38264 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers