Update libexpat to version 2.6.0.

This fixes CVE-2023-52425. OpenBSD is not affected by CVE-2023-52426.
Relevant for OpenBSD are security fixes #789 #814, bug fixes #753
#812 #813, other changes #7

Update libexpat to version 2.6.0.

This fixes CVE-2023-52425. OpenBSD is not affected by CVE-2023-52426.
Relevant for OpenBSD are security fixes #789 #814, bug fixes #753
#812 #813, other changes #771 #788 #764 #765, and examples, docs,
compiler warnings, clang-tidy, tests. Only a minor library bump
is necessary, this has been discussed with tb@ guenther@ kettenis@.

OK deraadt@

show more ...