TPA-RFC-67: retire mininag

in #41633 (closed), we identified that, with the NRPE retirement, mini-nag will break.

we need to figure out how exactly mini-nag works and how to deal with that retirement.

next steps:

• evaluate possibility of replacing or rewriting mininag (not practical)
• propose retirement (https://gitlab.torproject.org/tpo/tpa/team/-/wikis/policy/tpa-rfc-67-retire-mini-nag)
• wait for approval
• disable cron job
• hack reboot script to talk to auto-dns