Doc, build: Set minimum libxml2 version

The modern parser API (for example, xmlReadDoc) was added in libxml
v2.6.0:
https://gitlab.gnome.org/GNOME/libxml2/-/commit/16fa96c5

We already use this API, but we have not documented the version
requirement. This is not worrisome, since v2.6.0 was released 20 years
ago.

Ref T719

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>