Optimise member_roles.elevating related queries

What does this MR do and why?

In !114734 (merged) we merged adding read_vulnerabilitypermission to Custom roles. This, however, caused a performance problem. Afterward, we also added admin_vulnerability permission.

We need to improve the performance of elevating scope. Right now the query is behind a feature flag elevated_guests.

Database

I specifically tested the query called from billed_group_users for group gitlab-org as this one is used on the page we saw the problem on (group members page on gitlab.com).

• Old query: https://console.postgres.ai/shared/5beae453-e31f-4e78-8566-06e7a5ad2ea0
• New query: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/19924/commands/65068

MR acceptance checklist

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

Related to #411385 (closed)