rc.d/NETWORKING: remove the NETWORK alias

NETWORKING is the documented placeholder, while
/etc/rc.d/NETWORKING still provides the legacy alias
NETWORK.

The NETWORKING script was originally introduced to avoid
conflicts with NetBSD's lowercase network script on
case-insensitive file systems. The NETWORK alias was
retained for compatibility with older scripts.

Following the discussion in PR 293652, remove the legacy
NETWORK alias from 16-CURRENT. Keeping both names adds
more confusion than value now that NETWORKING is the
documented placeholder and current base system and ports
tree uses are already clean.

Add an UPDATING entry to note that local RC scripts using
REQUIRE: NETWORK should be migrated to REQUIRE: NETWORKING.

PR: 293652
Reviewed by: michaelo, jlduran
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D56300