remove old perl entries from ObsoleteFiles.inc

Each entry in ObsoleteFiles.inc adds to the time make delete-old and
friends take to run. Perl was removed from the FreeBSD base system a
very long time ago (FreeBSD 5); source updates have not been supported
from that version for years.

Perl was a single component responsible for thousands of entries so
provides significant benefit with little effort/investigation required.
We could still use a more comprehensive cleanup to remove old entries.

Also add an UPDATING note (with wordsmithing by imp) indicating that
make delete-old is required along each step of a source upgrade from
an old, unsupported release.

Discussed with: imp
Sponsored by: The FreeBSD Foundation