Clean up schema for table protected_branch_merge_access_levels

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Per 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
• Missing constraint: UNIQUE (protected_branch_id) (9782 entries violate this rule) (or perhaps with user_id? or group_id? or both?)