security/vuxml: reference FreeBSD SA-24:03.unbound

Add a reference to FreeBSD SA-24:03.unbound (announced 2024-03-28) to
the vuxml entry for Unbound CVE-2023-50387 and CVE-2023-50868.

Unbound was updated to 1.19.1 in FreeBSD 14.0-RELEASE-p6 and in FreeBSD
13.2-RELEASE-p11. FreeBSD 13.3 was not affected (Unbound 1.19.1 was
included in the release).