Add MR approval policy error message for `branch_type: default` when default branch is unprotected
Why are we doing this work
As a simplification of #444676 we want to add an error message to the policy editor UI if at least one rule of a policy sets branch_type: default, but the project's default branch isn't protected.
We only add an error message to the project-level policy editor, not the group-level one.
