Re-queue BackfillUserGroupMemberRoles

What does this MR do and why?

• Follows from a background migration created in !200163 (merged)
• The background migration failed for some records for two reasons
  • The user_id was null, which is the case for members that have been invited but have not accepted the invite
  • We have duplicate records for user, source in the members table because we don't have a unique index on them. I checked the records manually in the database and they have exactly the same data.
• This MR fixes both the issue and re-queues the migration

How to set up and validate locally

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Related to #520189 (closed)