TPA-RFC-10: adopt jenkins retirement plan

I've been brainstorming here and there about how we could eventually replace Jenkins. Now that people are getting excited about GitLab CI and that service is taking up more and more of our time, it seems like a good time to start planning for Jenkins retirement.

Requires jenkins service docs first (#40166 (closed)).

update: followup of this proposal is in https://gitlab.torproject.org/groups/tpo/-/milestones/27