Do not allow last human owner to leave group

What does this MR do and why?

Fixes #363387 (closed)

Exclude project_bot owners when checking if there is only one last owner for a group.

Also DRY up common logic for LastGroupOwnerAssigner. This was causing the UI to show button to be able to leave group when the backend does not allow it.

Screenshots or screen recordings

Before	After

How to set up and validate locally

See #363387 (closed)

MR acceptance checklist

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