security/vuxml: Document mod_gnutls -- multiple vulnerabilities

CVE-2026-33307 is an out-of-bounds write when mod_gnutls receives a client
certificate chain longer than its buffer, CVE-2026-33308 a missing Key
Purpose check during client certificate verification. Upstream fixed both
on 2026-03-20; in the tree the fix arrives with www/mod_gnutls 0.13.0.

Sponsored by: Netzkommune GmbH