Use xmlAddChildList not xmlAddChild in XMLSERIALIZE.

It looks like we should have been doing this all along,
but we got away with the wrong coding until libxml2 2.13.0
tightened up xmlAddChild's behavior.

There is more stuff to be fixed to be compatible with 2.13.0,
and it will all need to be back-patched.  This is just a
trial commit to see if the buildfarm agrees that we can use
xmlAddChildList unconditionally.

Erik Wienhold, per report from Frank Streitzig.

Discussion: https://postgr.es/m/trinity-b0161630-d230-4598-9ebc-7a23acdb37cb-1720186432160@3c-app-gmx-bap25
Discussion: https://postgr.es/m/trinity-361ba18b-541a-4fe7-bc63-655ae3a7d599-1720259822452@3c-app-gmx-bs01

src/backend/utils/adt/xml.c

@@ -757,7 +757,7 @@ xmltotext_with_options(xmltype *data, XmlOptionType xmloption_arg, bool indent)
 
 			/* This attaches root to doc, so we need not free it separately. */
 			xmlDocSetRootElement(doc, root);
-			xmlAddChild(root, content_nodes);
+			xmlAddChildList(root, content_nodes);
 
 			/*
 			 * We use this node to insert newlines in the dump.  Note: in at