psql \dX: reference regclass with "pg_catalog." prefix

Déjà vu of commit fc40ba1296, for another backslash command.
Strictly speaking this isn't a bug, but since all references to catalog
objects are schema-qualified, we might as well be consistent.  The
omission first appeared in commit ad600bba04 and replicated in
a4d75c86bf15; backpatch to 14.

Author: Justin Pryzby <pryzbyj@telsasoft.com>
Discussion: https://postgr.es/m/20210827193151.GN26465@telsasoft.com

src/bin/psql/describe.c

@@ -4735,7 +4735,7 @@ listExtendedStats(const char *pattern)
 		appendPQExpBuffer(&buf,
 						  "pg_catalog.format('%%s FROM %%s', \n"
 						  "  pg_get_statisticsobjdef_columns(es.oid), \n"
-						  "  es.stxrelid::regclass) AS \"%s\"",
+						  "  es.stxrelid::pg_catalog.regclass) AS \"%s\"",
 						  gettext_noop("Definition"));
 	else
 		appendPQExpBuffer(&buf,
@@ -4746,7 +4746,7 @@ listExtendedStats(const char *pattern)
 						  "   ON (es.stxrelid = a.attrelid \n"
 						  "   AND a.attnum = s.attnum \n"
 						  "   AND NOT a.attisdropped)), \n"
-						  "es.stxrelid::regclass) AS \"%s\"",
+						  "es.stxrelid::pg_catalog.regclass) AS \"%s\"",
 						  gettext_noop("Definition"));
 
 	appendPQExpBuffer(&buf,