release steps: stop announcing release candidates

Created by: slimsag

Today our release process https://github.com/sourcegraph/sourcegraph/issues/13374 has us announce to #dev-announce in Slack that release candidates have been created. This makes releases eventful and, I believe, gives the impression that there is some extensive testing of release candidates going on when in practice it is just basic smoke tests and all extensive testing should've been done before any change was merged.

I also believe that most team members do not actually care for or act on these messages, so they are spammy. We should stop sending them, remove the associated scripts (yarn run release release-candidate:dev-announce 3.20.0-rc.1) and steps from the release process tracking issue.