Make RemoteMirror's only_protected_branches default value consistent

From https://gitlab.com/gitlab-org/gitlab-ce/issues/53515, we see the backend appears to have inconsistent default values for this column:

• DB schema: false
• UI checkbox: false
• RemoteMirror model: true

This leads to unintended behavior where the boolean is activated if the UI doesn't pass in a value for the checkbox.