14.1: update stable/14 to -PRERELEASE

This marks the start of the FreeBSD 14.1 release cycle; the stable/14
tree is now in "code slush".

Developers are encouraged to prioritize fixing bugs (and/or merging bug
fixes from HEAD) over new features at this time. Commit approval from
re@ is not required but if new features introduce problems they may be
removed from the release.

Approved by: re (implicit)