main/libxml2: upgrade to 2.11.6

https://gitlab.gnome.org/GNOME/libxml2/-/compare/v2.11.5...v2.11.6 -> code changes lgtm

Changelog

v2.11.6: Nov 16 2023

### Regressions

- threads: Fix --with-thread-alloc
- xinclude: Fix 'last' pointer in xmlXIncludeCopyNode

### Bug fixes

- parser: Fix potential use-after-free in xmlParseCharDataInternal

https://gitlab.gnome.org/GNOME/libxml2/-/blob/93f0179d9d0bcd0d053d2df3e2fd382e6dac6954/NEWS