Check group last owner before assigning default membership role

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Fixes #367953 (closed)

SAML Group Sync does not check if a member is the last owner before assigning the default membership. In some cases this led to groups being without an owner. To avoid this the worker now checks for last owner before changing the member to default membership role.

The ideal solution is at a lower level so that this behavior is consistent across the platform. However, that is a rabbit hole at the moment. It will takes updates to potentially hundreds of specs. I created a follow-up issue for us to discuss the long-term solution in more depth. #368946

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