prop289: Update proposal with new deployment plan
New plan is in 3 phases:
- emit=v1, accept=v0, protover=v0
- emit=v1, accept=v0, protover=v1
- emit=v1, accept=v1, protover=v1
See https://trac.torproject.org/projects/tor/ticket/30364#comment:8 for the rationale.
This ticket is only for updating proposal 289 text.