Add DestroyInvalidMembers migration

What does this MR do and why?

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Issue: #375297 (closed)

We have some invalid membership records that may have been the result of an old console/SQL delete. The namespace IDs don't point to existing records. This MR removes them.

1. Add a temp index on null member namespace id so we can use it later: !99212 (merged) ✅
2. Delete the migration that used the less performant column: !99231 (merged)
3. Create new migration using the more performant column and index with a later timestamp !99467 (merged) (this MR)
4. Finalize migration !99080 (merged)
5. Drop the temp index

% rails db:migrate             
main: == 20220930062114 ScheduleDestroyInvalidMembers: migrating ====================
main: == 20220930062114 ScheduleDestroyInvalidMembers: migrated (0.0540s) ===========

main: == 20220930062114 ScheduleDestroyInvalidMembers: reverting ====================
main: == 20220930062114 ScheduleDestroyInvalidMembers: reverted (0.0678s) ===========

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