make it possible to test with synthetic source

Author: tteofili

qa/rolling-upgrade/src/javaRestTest/java/org/elasticsearch/upgrades/DenseVectorMappingUpdateIT.java

@@ -88,7 +88,13 @@ public void testDenseVectorMappingUpdateOnOldCluster() throws IOException {
                 Request createIndex = new Request("PUT", "/" + indexName);
                 XContentBuilder mappings = XContentBuilder.builder(XContentType.JSON.xContent())
                     .startObject()
-                    .startObject("mappings")
+                    .startObject("mappings");
+                if (randomBoolean() && getOldClusterTestVersion().after(Version.V_8_12_0.toString())) {
+                    mappings = mappings.startObject("_source");
+                    mappings.field("mode", "synthetic");
+                    mappings.endObject();
+                }
+                mappings = mappings
                     .startObject("properties")
                     .startObject("embedding")
                     .field("type", "dense_vector")