Merge branch 'mg-cachekeyfiles-variables-20240124' into 'master'

Warn about variable usage in cache:key:files

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/142690



Merged-by: Lysanne Pinto <lpinto@gitlab.com>
Approved-by: Lysanne Pinto <lpinto@gitlab.com>
Co-authored-by: Manuel Grabowski <mgrabowski@gitlab.com>