Resolve "Unable to view projects pending deletion from subgroup"

What does this MR do and why?

Related to #398694 (closed)

History

We have tried to solve this problem recently in !119292 (diffs), but it had to be reverted due to an incident in #409825 (closed).

I have tried to elaborate the root cause of the incident (a query timeout) in !119292 (comment 1379089701), TL;DR is that we were querying projects across all paid groups and subgroups on gitlab.com database, which was bound to timeout.

With this MR, we are trying to limit the number of groups being queried, which is showing promising results in the query plan.

Query plans:

Using gitlab-qa user:

• Current query plan, based on the change in this MR: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/18400/commands/60908
• Query plan of this scope in current master : https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/18449/commands/61138
• Query plan of the change we attempted in !119292 (diffs), which times out: https://postgres.ai/console/gitlab/gitlab-production-tunnel-pg12/sessions/18449/commands/61139

When fixed, this would solve the bug as described in the description of !119292 (merged)

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Related to #398694 (closed)