UPDATING: Add note for java/openjdk{25,26} package renames

The jre-flavored packages were renamed in ad618043f3d. Instruct users
on how to rename their installed packages, so pkg upgrade will
correctly replace them with the new package names.

Reviewed by: haraldei
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56123