sysutils/cpu-microcode-rc: Update pkg-descr and pkg-message

• Note that the early loading is now available for AMD systems after 2024-02-22 src snapshot, 34467bd76.
• Do not refer to old port names
• Refer to sysutils/cpu-microcode-* so we are covered if any new CPU microcode ports are added to the tree

Reviewed by: chs
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D44048