[3.18] main/libxml2: upgrade to 2.11.6

related to !55569 (merged), because parser: Fix potential use-after-free in xmlParseCharDataInternal

https://gitlab.gnome.org/GNOME/libxml2/-/compare/v2.11.4...v2.11.6

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


v2.11.5: Aug 9 2023

### Regressions

- parser: Make xmlSwitchEncoding always skip the BOM
- autotools: Improve iconv check

### Bug fixes

- valid: Fix c1->parent pointer in xmlCopyDocElementContent
- encoding: Always call ucnv_convertEx with flush set to false

### Portability

- autotools: fix Python module file ext for cygwin/msys2 (Christoph Reiter)

### Tests

- runtest: Fix compilation without LIBXML_HTML_ENABLED

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