Add comment about Golang and FIPS

What does this MR do and why?

This MR adds a comment to the README.md (and scripts/lib/custom-docker-build) explaining that when adding a new major/minor Golang version, it may be necessary to use an old golang-fips branch.

Related: Also install Go 1.22 (!771 - merged)

Checklist

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