13.4: update stable/13 to -PRERELEASE

This marks the start of the FreeBSD 13.4 release cycle; the stable/13
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)
Sponsored by: Amazon