Cells: Fix cross-join between namespaces, redirect_routes & routes

(some) failing specs, for reference:

All cross-joins that are currently allowed to pass in the above failing specs should be fixed. They are marked in the codebase with allow_cross_joins_across_databases(url: "https://gitlab.com/gitlab-org/gitlab/-/issues/420046")

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
Edited by Manoj M J