Rewrite the sections about updating and testing. Mention CVSup and advise
checking for already-pending updates in GNATS and/or portsmon in the
updating section. Move the FreshPorts text (formerly specific to testing)
to a more general Keeping Up section, and add text about CVSWeb, the
freebsd-ports mailing list, the build cluster, the distfile survey, and
portsmon.

PR: docs/66855
Approved by: ceri (mentor)