Move organization_users from gitlab_main_clusterwide to gitlab_main_cell

Summary

Due to the decision in #419366 (closed) to have organization_users in the main cell, we need to now move it.

It currently has no data in any environments, so this will mean we need to:

• Move the table in the gitlab_schema
• Covert any needed foreign keys to loose foreign keys
  • guide: https://docs.gitlab.com/ee/development/database/loose_foreign_keys.html

original info

The following discussion from !139188 (merged) should be addressed:

• @dstull started a discussion: (+1 comment)

  note (non-blocking):

  @alexpooley @abdwdd I'm not sure if this is the right way to handle this or if anything else needs changed here... any guidance? Doing this due to the failure described below and this discussion

  Without it, of course the cross database joins alert fires - https://gitlab.com/gitlab-org/gitlab/-/jobs/5787672213#L1967

  now with current code it fails for foreign keys in a different way: https://gitlab.com/gitlab-org/gitlab/-/jobs/5788183962#L9442

  Any ideas how we might usually solve this one? Should we deal with a process to move organization_users to gitlab_main_cell first? I assume we may have a process for that? It seems like it wants me to move organizations then to gitlab_main_cell too...but I'm not sure that is right.