Cells: Fix cross-join between namespaces & plans
All cross-joins that are currently allowed to pass should be fixed. They are marked in the codebase with allow_cross_joins_across_databases(url: "https://gitlab.com/gitlab-org/gitlab/-/issues/419988")
Refer to the epic for more information about this. Feel free to:
- Copy the epic description to this issue to helper the reviewers understand the problem.
- Split this issue into different issues in case it was so much effort to
- Ask for help and hints on #g_tenant-scale on how to fix cross joins
Edited by Manoj M J [PTO - Back on 17th June]