message: update release process for the move to src/version
diff: Here's how Rust is currently released:
  -## Promote beta to stable (T-6 days, Friday the week before)
  +## Bump the stable version number (T-6 days, Friday the week before)
  -Open a PR bumping the version number in `src/bootstrap/channel.rs`. r+
  -rollup=never this PR.
  +Open a PR bumping the version number in `src/version`. `r+ rollup=never` this
  +PR.
  -Mark it as rollup=never, because if it lands in a rollup as *not* the first PR
  -then other pull requests in that rollup will be incorrectly associated with the
  -prior release.
  +Mark it as `rollup=never`, because if it lands in a rollup as *not* the first
  +PR then other pull requests in that rollup will be incorrectly associated with
  +the prior release.
  ## Promote beta to stable (T-3 days, Monday)
  