Latest and minimum supported version of every Shardflux client

Unauthenticated, Cache-Control: public, max-age=3600. Clients compare their own version: below minimum_supported they are unsupported, below the latest release outdated. Current clients take the latest release from the package registry (npm latest, PyPI) and use latest here as the fallback; hold names a release they stop recommending. They check at most every 12 hours (the CLI) or once per process (the SDKs), in the background, and never fail because of it.