Initial set of changes for v21 release

[rohit-nayak-ps]

Description

Mods required for v21 release:

• Change copy for new method of running local install, due to: Handle latest version downloads programmatically website#1855. Change from having to add/replace hash of release commit, to specifying version for previous version's local install page.
• Use bot to create PRs that are generated by the tool, so that we only need two developers online for approving PRs.
  The person running the releaser should create a Personal Access Token with repo rights in the Vitess repo. Define a environment variable calledVITESS_RELEASER_GH_TOKEN set to the token.
• Remove -rc from target branch name for releases. All releases will be on the release-xx.0 branch, starting v21

[frouioui]

Left a comment, otherwise looks good to me!