Merge branch 'xx/ensure-featureflag-always-in-lowercase' into 'master'

featureflag: Ensure to only contain valid characters

See merge request https://gitlab.com/gitlab-org/gitaly/-/merge_requests/4793



Merged-by: John Cai <jcai@gitlab.com>
Approved-by: Toon Claes <toon@gitlab.com>
Approved-by: John Cai <jcai@gitlab.com>
Co-authored-by: blanet <moweng.xx@alibaba-inc.com>