[TRANSFORM] No point sorting preview docs for serialization (#94611)

Currently we serialize the transform preview docs in sorted order.
However, when we read them at the other end we deserialize into a
hash map, which loses the ordering. Therefore sorting when
serializing is pointless.

The serverless integration tests are tripping an assertion in
writeMapWithConsistentOrder that the map being serialized is not
a linked hash map. It makes no difference to the functionality
if it is, so this does not affect production clusters (where
assertions are disabled). But it causes integration tests to fail.

Author: droberts195

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/transform/action/PreviewTransformAction.java

@@ -200,7 +200,7 @@ public TransformDestIndexSettings getGeneratedDestIndexSettings() {
         public void writeTo(StreamOutput out) throws IOException {
             out.writeInt(docs.size());
             for (Map<String, Object> doc : docs) {
-                out.writeMapWithConsistentOrder(doc);
+                out.writeGenericMap(doc);
             }
             generatedDestIndexSettings.writeTo(out);
         }