Cells: Iteration 2: Identify all cross-joins between `users` and `namespaces`

Problem

As part of the Cells: Iteration 2: Make `users` to be `gitlab_... (#410370 - closed) and Cells: Iteration 2: Make `namespaces` to be `gi... (#390102 - closed) the users and namespaces become part of separate schemas. It is not allowed to cross-join across separate databases.

Currently those schemas are allowed to cross-join with due db/gitlab_schemas/gitlab_main_cell.yaml having allow_cross_joins: gitlab_main_clusterwide allow.

Steps

We should identify all cross-joins by:

1. From db/gitlab_schemas/gitlab_main_cell.yaml removing allow_cross_joins: gitlab_main_clusterwide.
2. Trying to get pipeline green, by:
   • get pipeline green by using allow_cross_joins_across_databases, and creating associated issue
   • allowlisting temporary all failed specs in spec/support/database/cross-join-allowlist.yml
3. Once identified schedule work on issues that overlap with Cells: Iteration 2: Fix all cross-joins related... (#409842 - closed)