prop289: Update proposal with new deployment plan

New plan is in 3 phases:

1. emit=v1, accept=v0, protover=v0
2. emit=v1, accept=v0, protover=v1
3. 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.