Thanks to T264343 / structured mentor list, we now have an "umbrella API endpoint" to manage information about mentors. Once the structured mentor list gets deployed (T310905), we should replace `action=growthsetmentorstatus` with `action=growthmanagementorlist`.
Checklist:
[X] Complete {T321502}
[X] Remove all internal callers of `action=growthsetmentorstatus` ([r936090](https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GrowthExperiments/+/936090))
[X] Mark `action=growthsetmentorstatus` as deprecated, pointing users to `action=growthmanagementorlist` instead ([r936090](https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GrowthExperiments/+/936090))
[ ] Announce change via #user-notice
[ ] Wait a week & check for usages. Either remove code supporting `action=growthsetmentorstatus` or re-announce, depending on how often the API is actually used.