push back jenkins retirement deadline, clarify delays okay
- it's still in 2021 so we don't change the roadmap, and
- 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. ;)