security/vuxml: Fix affected port version of mail/thunderbird by CVE-2026-6778

Although mail/thunderbird does not actually have PORTEPOCH, one is included in vuxml.
As a result, even fixed versions were still incorrectly reported as vulnerable.

The incorrect report was as follows:

> thunderbird-153.0.2 has known vulnerabilities:

thunderbird-153.0.2 is vulnerable:

Mozilla -- Invalid pointer
CVE: CVE-2026-6778
WWW: https://vuxml.FreeBSD.org/freebsd/5124ce36-430a-11f1-a627-b42e991fc52e.html