Clean up schema for table protected_branch_push_access_levels
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) (11238 entries violate this rule) (or perhaps with user_id? or group_id? or both?)