textproc/expat2: [security] upgrade to 2.8.1
See https://blog.hartwork.org/posts/expat-2-8-1-released/
Fixes: CVE-2026-45186 and CVSS unreliability
Security: CVE-2026-45186
MFC after: 1 day
PR: 295206
Approved by: vishwin
(cherry picked from commit 709eee0a0c626886efbef572134e0faaae3eece6)