pg_dump: Allow dumping default collation

This will not work on restore, but it will allow dumping out pg_catalog
for research and documentation.

Reported-by: Neil Anderson <neil.t.anderson@gmail.com>
Bug: #14701

src/bin/pg_dump/pg_dump.c

@@ -13073,6 +13073,9 @@ dumpCollation(Archive *fout, CollInfo *collinfo)
 		appendPQExpBufferStr(q, "libc");
 	else if (collprovider[0] == 'i')
 		appendPQExpBufferStr(q, "icu");
+	else if (collprovider[0] == 'd')
+		/* to allow dumping pg_catalog; not accepted on input */
+		appendPQExpBufferStr(q, "default");
 	else
 		exit_horribly(NULL,
 					  "unrecognized collation provider: %s\n",