"Your connection to Tor is not being managed by Tor Browser" in about:tor can sound alarming

Reported in Joshua's interview](https://tails.net/contribute/how/user_experience/interviews/joshua/):

The page in Tor Browser that says "Your connection is not managed by Tor Browser". That didn't used to be there. When I saw it for the first time, I wondered what happened. That's kind of alarming.

Right now, it's only displayed in new tab but it will get more exposure with #21003 (closed).

When we switch to about:tor as homepage, it would be nice to customize this message in Tails to be less scary while still offering the link to https://check.torproject.org/.