xml2: Replace deprecated routines with recommended ones

xml2: Replace deprecated routines with recommended ones

Some functions are used in the tree and are currently marked as
deprecated by upstream.  This commit refreshes the code to use the
recommended functions, leading to the following changes:
- xmlSubstituteEntitiesDefault() is gone, and needs to be replaced with
XML_PARSE_NOENT for the paths doing the parsing.
- xmlParseMemory() -> xmlReadMemory().

These functions, as well as more functions setting global states, have
been officially marked as deprecated by upstream in August 2022.  Their
replacements exist since the 2001-ish area, as far as I have checked,
so that should be safe.

Author: Dmitry Koval
Discussion: https://postgr.es/m/18274-98d16bc03520665f@postgresql.org