Set schema of tracking tables to gitlab_shared

What does this MR do and why?

Related to #358893 (closed)

Update the gitlab_schema of tracking tables for background migrations. They need to be marked as gitlab_shared since tables will be present on both main and ci, and intended to be used in migrations for both gitlab_main and gitlab_ci.

Also changed the table postgres_reindex_queued_actions to be gitlab_shared. This is part of the automatic reindexing feature, which is also a cross-database feature. The other reindexing tables are already marked as gitlab_shared.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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