TPA-RFC-80: (make a) Debian trixie upgrade plan

the freeze dates have been announced:

2025-03-15      - Milestone 1 - Transition and toolchain freeze
2025-04-15      - Milestone 2 - Soft Freeze
2025-05-15      - Milestone 3 - Hard Freeze - for key packages and
                                packages without autopkgtests
To be announced - Milestone 4 - Full Freeze

it would be nice to start upgrading during the freeze.

in %Debian 12 bookworm upgrade we did 3 batches of upgrades, with the first two batches separated by a month of testing (september/october 2023). the second batch trailed until January 2023, that said, so we should also take this into account. We also had to prioritized the %Debian 11 bullseye upgrade which delayed the bookworm upgrade.

in any case, make a plan:

• who should do the upgrade? (e.g. should we loop the tails people in? 5 people is probably too much, but it might be nice to loop in the tails folks as it's good training)
• what do we upgrade when? (the batches system worked well this time again, let's do it again), sub questions:
  • do we also count the tails machines in that process? if so, do we schedule them in separate batches?
  • how do we process the infamous third batch? the ganeti upgrade trailed for a long time this time, for example, maybe even needlessly... we also ended up creating issues for each of those special snowflakes, perhaps it's best if we do that in advance? or do we schedule them to actually upgrade in batches (e.g. one batch for ganeti, one batch for eugeni/ldap/etc?)
• what is our timeline? we should start the upgrade during the freeze, do we want to finish it then as well?
• do we automate this more? we have automation issues opened here that have been dragging around for a long time, do we bite the bullet and move our procedure into fabric?

schedule in two weeks to give at least a month's notice to people if we start the upgrades in march, ideally two if we start during the soft freeze in april.

the result should be a RFC that looks something like https://gitlab.torproject.org/tpo/tpa/team/-/wikis/policy/tpa-rfc-57-bookworm-upgrades

note that a lot of the steps in the plan are already detailed in the milestone, %Debian 13 trixie upgrade

update: proposed in https://gitlab.torproject.org/tpo/tpa/team/-/wikis/policy/tpa-rfc-80-debian-trixie-upgrade-schedule