[ML] Further mutes on DFA upgrade tests

Because of elastic#64503. This adds to elastic#64540.

Author: dimitris-athanasiou

x-pack/qa/rolling-upgrade/src/test/resources/rest-api-spec/test/upgraded_cluster/90_ml_data_frame_analytics_crud.yml

@@ -81,6 +81,9 @@
 
 ---
 "Get mixed cluster outlier_detection job":
+  - skip:
+      version: " - 7.99.99"
+      reason: "_doc_count field is currently implemented only in 8.x"
   - do:
       ml.get_data_frame_analytics:
         id: "mixed_cluster_outlier_detection_job"
@@ -99,6 +102,9 @@
 
 ---
 "Get mixed cluster outlier_detection job stats":
+  - skip:
+      version: " - 7.99.99"
+      reason: "_doc_count field is currently implemented only in 8.x"
   - do:
       ml.get_data_frame_analytics_stats:
         id: "mixed_cluster_outlier_detection_job"