contrib/expat: import expat 2.7.1

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

https://github.com/libexpat/libexpat/blob/R_2_7_0/expat/Changes

Security: CVE-2024-8176

(cherry picked from commit fe9278888fd4414abe2d922e469cf608005f4c65)