2.12 migration tips

Just leaving this here, I've seen many feedstocks get hung up on the error handler not being declared `const`.

```patch

diff -ur a/src/backend/utils/adt/xml.c b/src/backend/utils/adt/xml.c
--- a/src/backend/utils/adt/xml.c 2023-11-06 17:04:27.000000000 -0500
+++ b/src/backend/utils/adt/xml.c 2023-12-03 21:01:12.452541626 -0500
@@ -123,7 +123,7 @@
xmlParserCtxtPtr ctxt);
static void xml_errsave(Node *escontext, PgXmlErrorContext *errcxt,
int sqlcode, const char *msg);
-static void xml_errorHandler(void *data, xmlErrorPtr error);
+static void xml_errorHandler(void *data, const xmlError *error);
static int errdetail_for_xml_code(int code);
static void chopStringInfoNewlines(StringInfo str);
static void appendStringInfoLineSeparator(StringInfo str);
@@ -2002,7 +2002,7 @@
* Error handler for libxml errors and warnings
*/
static void
-xml_errorHandler(void *data, xmlErrorPtr error)
+xml_errorHandler(void *data, const xmlError *error)
{
PgXmlErrorContext *xmlerrcxt = (PgXmlErrorContext *) data;
xmlParserCtxtPtr ctxt = (xmlParserCtxtPtr) error->ctxt;
```

Start by reading the migration note and the cited src/backend/utils/adt/xml.c diff, then check whether this feedstock has a documented location for 2.12 migration guidance. Done means the const error-handler requirement is recorded clearly for maintainers.

Written by the indexing model from the issue text.