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.