Behave similar to what an away mentor looks like, but with a message saying "mentor is unavailable" instead of "away".
Needs to be handled in conjunction with:
T389561: Mentorship: Provide context for replacement mentors when they receive a newcomer's message
Given a Mentor has a protected talk page,
When a newcomer asks them a question,
Then we treat the Mentor as "Away" and the question is directed to a "replacement mentor"
Rather than "away" the Mentor should be considered "unavailable".