TPA-RFC-71: make emergency email deployment plans, again

in #41773 (closed), we had yet another report of a problem with mail delivery, particularly with email forwards, that are plaguing gmail-backed aliases like grants@ and travel@.

this is becoming critical: it's been impeding people's capacity of using their email at work for a while, but it's been more acute since google's recent changes in email validation, as now hosts that have adopted the SPF/DKIM rules are bouncing.

on top of that, we're way behind on our buster upgrade schedule. we still have to upgrade eugeni, and the plan for that (TPA-RFC-45, #41009) was to basically rebuild everything and oh please add mailboxes too in there #ktxbye. that won't happen fast enough for the LTS retirement which we have crossed two months ago (in July 2024) already.

so, in essence, our main mailserver is unsupported now, and we need to fix this ASAP.

make yet another contingency plan to deploy emergency fixes to address out of date software and forwarding issues, ASAP.

• draft proposal
• review TPA-RFC-45
• background
• tasks
• scope
• affected users
• persona
• deadline
• timeline
• send proposal, wait a couple days, see https://gitlab.torproject.org/tpo/tpa/team/-/wikis/policy/tpa-rfc-71-emergency-email-deployments-round-2
• delegate issues, make milestone (%TPA-RFC-71: emergency email deployments, phase B)
• announce changes to tor-project