sysutils/cfengine*-lts: cfengine* 3.24.0 is LTS

cfengine and cfengine-masterfiles 3.24.0 are LTS.

Also adjust the documented versions one can choose. cfengine/masterfiles
3.21 upstream support will expoire at the end of 2025. While 3.24
upstream support will expire at the end of June 2027. Users can put
CFENGINE_VERSION=321 into their make.conf.

Please open a PR if people want both LTS versions supported by binary
updates. In that case we can also keep the old LTS version under a
different LTS port name.