Cells: Fix cross joins related to projects applicable_approval_rules_for_user

Fix the cross joins in these files:

1. ee/app/models/ee/project.rb:707: in applicable_approval_rules_for_user method

Refer to the epic for more information about this. Feel free to:

1. Copy the epic description to this issue to helper the reviewers understand the problem.
2. Split this issue into different issues in case it was so much effort to
3. Ask for help and hints on #g_tenant-scale on how to fix cross joins

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

Availability and Testing

Regression testing, ensure associated MR is labelled with pipeline:run-all-e2e and the e2e:package-and-test job is passing.