The teleport.MinClientVersion was also updated to be derived from
the new semver variant as well as adding a semver.Version variant.
Additionally instances of semver.New(teleport.Version) or
semver.New(api.Version) were replaced to use the newly introduced
variables. It was also noticed that auth.MinSupportedModeratedSessionsVersion
was no longer being used so it was removed.
Update master as the now-source of v16 when it gets cut:
* Update `VERSION` in `Makefile` to 16.0.0-dev
* Update `BUILDBOX_VERSION` in `build.assets/Makefile` to `teleport16`
* Update a couple of mentions of v13 in `README.md` to v16
* Run `make update-version`
* Run `make dronegen`
Some workflows are to be updated once v15.0.0 is actually released.