Forwarded request #1189926 from darix

- 1.16.6:
- update intree libxml2 copy (unused in our build)
- update intree zlib copy (unused in our build)
- [CRuby] `XML::Reader` sets the `@encoding` instance variable
during reading if it is not passed into the initializer.
Previously, it would remain `nil`. The behavior of
`Reader#encoding` has not changed. This works around changes to
how libxml2 reports the encoding used in v2.12.6.
- [CRuby] `XML::Reader` defaults the encoding to UTF-8 if it's
not specified in either the document or as a method parameter.
Previously non-ASCII characters were serialized as NCRs in this
case. [#2891] (@flavorjones)
- [CRuby] Restored support for compilation by GCC versions
earlier than 4.6, which was broken in v1.15.0 (540e9aee).
[#3090] (@adfoster-r7)
- [CRuby] Patched upstream libxml2 to allow parsing HTML5 in the
context of a namespaced node (e.g., foreign content like
MathML). [#3112, #3116] (@flavorjones)
- [CRuby] Fixed a small memory leak in libgumbo (HTML5 parser)
when the maximum tree depth limit is hit. [#3098, #3100]
(@stevecheckoway)
- Drop gcc14-allow-incompatible-pointer-types-for-now.patch