Cells: Fix cross joins in epics_finder

Fix the cross joins in these files:

• ee/app/finders/epics_finder.rb:57:

Refer to the epic for more information about this.

Why cross joins are not allowed between users and namespaces?

The table users is planned to be moved to the gitlab_main_clusterwide database. Which means that it wont reside on the same database where namespaces table is going to be.

Why do we have this epic?

As part of the MR: !124319 (merged), we identified all the cross joins between the tables users and namespaces, and marked all the associations or code blocks to temporarily allow those violations with issues to fix them.

How to approach each one of the issues in this epic?

• Remove the allow_cross_joins_across_databases that are referenced in each issue, and see which tests are failing.
• Refer to this documentation on how to resolve the cross joins https://docs.gitlab.com/ee/development/database/multiple_databases.html#removing-joins-between-ci-and-non-ci-tables
• Remove the allowed cross joins, and make sure the pipeline is still green