remove empty cask, use temurin[at]21

[andres-lowrie]

When following this guide today I got back the following errors:

brew tap homebrew/cask-versions
Error: homebrew/cask-versions was deprecated. This tap is now empty and all its contents were either deleted or migrated.

and then

 brew install --cask temurin21
Warning: Cask 'temurin21' is unavailable: No Cask with this name exists.

==> Searching for similarly named casks...
==> Casks
temurin               temurin@11            temurin@17            temurin@19            temurin@20            temurin@21            temurin@8

pointing to temurin@21 works, hence that change

• [x] Have you read the guidelines for contributing?
• [x] Have you signed the Clojure Contributor Agreement?
• [x] Have you verified your asciidoc markup is correct?

[puredanger]

Would be happy to merge, but need the contributor agreement fully completed first.

[andres-lowrie]

Would be happy to merge, but need the contributor agreement fully completed first.

just resubmitted it