contrib/expat: update libexpat from 2.6.0 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. None of the vulnerabilities addressed by expat
releases 2.6.1 - 2.7.1 is exploitable on FreeBSD as supported by the
security-officer@ team.

Approved by: so
Security: FreeBSD-EN-25:05.expat
Security: CVE-2024-8176
Security: CVE-2024-50602
Security: CVE-2024-45490, CVE-2024-45491, CVE-2024-45492
Security: CVE-2024-28757

(cherry picked from commit ffd294a1f4c23863c3e515d16dce31d5509bcb01)
(cherry picked from commit 7fcc9d60956fc482d14acd579786390b989a5057)
(cherry picked from commit ba23ab2168ffabc2c5e647a1a37ab9a8fb482bb8)
(cherry picked from commit 8c7e4d11fef6ba9d5fc26571f72b8d877b0c1efb)
(cherry picked from commit 908f215e80fa482aa953c39afa6bb516f561fc00)
(cherry picked from commit 78ab4c182b9faa09688a040b252ba111952e5e17)
(cherry picked from commit fe9278888fd4414abe2d922e469cf608005f4c65)
(cherry picked from commit 6f7ee9ac036ebd210d70cb177eba0c3c5bc930e3)
(cherry picked from commit 03a1992591b0ae85b6b250255fe56e17f6d919c6)
(cherry picked from commit 50c58014795c63813c508bce59c97895ae2ca3c7)
(cherry picked from commit 00c8538e87c61f1fd57ccd9e02a6d435b68d9a73)
(cherry picked from commit fd4592006b1306e1d63a542612fd66a8374d181f)