Add Go 1.21 Operator build image

What does this MR do and why?

Adds a new Operator build image with Go 1.21 and the Operator tools. The 1.20 image remains for now for possible backport to stable branches.

Relates gitlab-org/cloud-native/gitlab-operator#1500 (closed)

Checklist

• (If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.