contrib/expat: update libexpat from 2.6.4 to 2.7.1

Changes: https://github.com/libexpat/libexpat/blob/R_2_7_1/expat/Changes

Note that libbsdxml(3) is only intended to used by utilities in the
FreeBSD base system. The vulnerability addressed by expat 2.7.0 is not
exploitable on FreeBSD as supported by the security-officer@ team.

Approved by: so
Security: FreeBSD-EN-25:05.expat
Security: CVE-2024-8176

(cherry picked from commit fe9278888fd4414abe2d922e469cf608005f4c65)
(cherry picked from commit 41b768ae1970ed484abaaea401453c3902df93c2)
(cherry picked from commit 03a1992591b0ae85b6b250255fe56e17f6d919c6)
(cherry picked from commit adc9e9e8dbddcf7d57bcdef0d9d0a0e7c08c15ba)
(cherry picked from commit 00c8538e87c61f1fd57ccd9e02a6d435b68d9a73)
(cherry picked from commit 5630672e6f6d58597a3d6f01928a7703f1cdd207)