15.1/relnotes: Cleanups

Reword mention of C23 support from "Partial support is now available"
to "Expanded support is now available" since 15.0 had some parts of
C23 already.

Remove mention of enabling padlock_rng, since the entire "disable and
then re-enable" cycle happened since 15.0.

Remove mention of taking KDE out of the dvd1 image, since that took
place in 15.0 as well.

Remove mention of shipping firmware packages from the kmods repo on
the DVD image, since that change was reverted.

Remvoe mention of ext2 and ntfs packages, since that took place in
15.0 as well.

Remove sections which are now empty.

Reviewed by: dch
Differential Revision: https://reviews.freebsd.org/D57602