Remove User cross-join from user_management.rake

What does this MR do and why?

Refactor and remove the cross-join in the query for users in user_management.rake

• Old query: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/20437/commands/66828
• New query: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/20437/commands/66868

Context: I extracted this out of !126559 (merged) since this isn't really related and makes things easier to review.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #417455 (closed)