Merge branch 'pguinoiseau/update-cells-diff-secrets-management' into 'master'

Update the secrets management section in the Cells Infra blueprint

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/148612



Merged-by: Steve Xuereb <sxuereb@gitlab.com>
Approved-by: Adeline Yeung <ayeung@gitlab.com>
Approved-by: Steve Xuereb <sxuereb@gitlab.com>
Co-authored-by: Pierre Guinoiseau <pguinoiseau@gitlab.com>