push back jenkins retirement deadline, clarify delays okay

September seemed a bit early to @hiro and @gaba, push it back three months. We pick December 1st because:

1. it's still in 2021 so we don't change the roadmap, and
2. it's a wednesday, so a good time to shutdown a service (ie. not a weekend or friday)

We also clarify that we're not fascists: we won't destroy the world if people haven't managed to respect the deadline.

We drop the time estimate for the migration, because that's actually a changing number. We were counting from "right now" which obviously changes every day. ;)