Clean up schema for table approver_groups

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From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

• Wrong data type: created_at should be timestamptz
• Wrong data type: updated_at should be timestamptz
• "Polymorphic" table prevents regular FKs, use triggers or change to independent columns or non-polymorphic tables
• Missing constraint: created_at NOT NULL
• Missing constraint: updated_at NOT NULL

We should replace target_type and target_id with merge_request_id and project_id, both with a foreign key to the right table. We also need to add a Rails validation that ensures only one of the two can be set for every row.