Pin bridges.torproject.org domains to Let's Encrypt's root cert public key
As discussed in https://gitlab.torproject.org/tpo/tpa/team/-/issues/41123#note_2896442 and above.
See also [some pinning for other torproject.org domains](https://gitlab.torproject.org/search?search=kPinset_tor&nav_source=navbar&project_id=472&group_id=477&search_code=true&repository_ref=tor-browser-102.10.0esr-12.5-1) which Firefox already appears to do.
/cc @micah, @richard
issue