clarify tails merge roadmap and split in milestones

In the wiki we have a timeline (and elsewhere in that document, a roadmap) for a merge with Tails, which has now been approved by the board.

Review the roadmap with @zen, @groente and possibly @micah and the other @tpo/tpa folks. For this we need to split things up into priorities, decide how we'll decide things, what we'll merge, drop, keep, etc, and split that up in years and quarters, and ultimately GitLab tickets and milestones. I suspect @gaba could be of precious help in organizing this as well.

This is marked as due on October 1st, but our tentative timeline marks "end of Q4 2024" as the deadline. To finish that by that time, however, I'll need a kick and I think starting in October is a good idea.

Onboarding of the tails folks in TPA is tracked elsewhere (#41720 and #41719, specifically).

To-do

• Identify dependencies between service merges
• Agree on a roadmap: TPA-RFC-73