[Synthtrace] Fix synthtrace to work with 7.x clusters (elastic#209447)

## Summary

While working on 9.x Upgrade testing, i found that when running
synthtrace scenarios pointing to a 7.x cluster, it tries to fetch the
latest version of the APM package. In 7.x, the `latestVersion` item was
present under a different node in the API response. Hence adding this
fix.

packages/kbn-apm-synthtrace/src/lib/apm/client/apm_synthtrace_kibana_client.ts

@@ -53,6 +53,11 @@ export class ApmSynthtraceKibanaClient {
         );
       }
 
+      // Add support for 7.x stack as latest version is available under different node
+      if (responseJson.response && responseJson.response.latestVersion) {
+        return responseJson.response.latestVersion as string;
+      }
+
       return responseJson.item.latestVersion as string;
     };