ci: Start using external secret for ADD_JH_FILES_TOKEN
What does this MR do and why?
After the successful PoC that we made at !143201 (closed), we're ready to start using external secrets from GSM.
We start with a single token: ADD_JH_FILES_TOKEN for now.
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.
Related to gitlab-org/quality/engineering-productivity/team#363.
Edited  by Rémy Coutable