This PR updates the release process following up on the addition of the
bump-stage0 tool in the rust-lang/rust monorepo, making these changes:

* Downloading artifacts from dev-static.rust-lang.org is not supported
  anymore, so the step to bump stage0 during beta promotion is removed.
  This doesn't have any practical effect, as we're still building the
  new beta with the previous beta.

* As there is no dependency anymore between promoting beta and promoting
  stable both are moved to the same day, bringing the master bootstrap
  update one day earlier.

* The step to update the beta bootstrap compiler after the release was
  updated to use the tool.