Rename source group associations to groups
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We have temporally renamed the groups and group associations to `source_groups` and `source_group` respectively.
We have also exposed `approver_users` and `approver_groups` as `users` and `groups` respectively as this is how they are named in the v1 architecture.
When all v1 callers of these methods have been updated to use the new method names we should remove these temporary names in the MergeRequests::ApprovalRule model.
issue