Fix deprecation warning with libxml2 2.14 in contrib/xml2/

Hi all,

A colleague, Walid Abrahim, has reported that contrib/xml2/ gives a
deprecation warning when attempting to compile xpath.c with upstream
libxml2 2.14 (just checkout the top of branch 2.14):
xpath.c: In function ‘pgxmlNodeSetToText’:
xpath.c:212:17: warning: ‘content’ is deprecated [-Wdeprecated-declarations]
  212 |                 result = xmlStrdup(buf->content);
      |                 ^~~~~~
In file included from /home/pooh/stable/libxml/2.14/include/libxml/parser.h:16,
                 from /home/pooh/stable/libxml/2.14/include/libxml/tree.h:17,
                 from /home/pooh/stable/libxml/2.14/include/libxml/xpath.h:27,
                 from xpath.c:19:
/home/pooh/stable/libxml/2.14/include/libxml/tree.h:103:14: note: declared here
  103 |     xmlChar *content XML_DEPRECATED_MEMBER;
      |              ^~~~~~~

This is caused by the following commit, where upstream is deciding
that we should not directly look at the "content" field of an
xmlBuffer:
https://github.com/GNOME/libxml2/commit/b34dc1e4

The solution is simple: we need to replace buf->content by a call to
xmlBufferContent().  This routine exists since be803967dbec (year
2000), so 25 years should make that safe enough to use.

The last batch of libxml deprecation warning fixes we have done was
last year in [1], so the one we have here is new.  It seems to me that
it would not hurt to just backpatch that all the way down rather than
waiting.  Comments are welcome.

[1] https://www.postgresql.org/message-id/1012981.1713222862@sss.pgh.pa.us

Regards,
--
Michael