[ML] Unmute DF Analytics BWC tests and fix versions for memory_usage (#52981)

Relates #52778

Author: dimitris-athanasiou

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/action/GetDataFrameAnalyticsStatsAction.java

@@ -192,7 +192,7 @@ public Stats(StreamInput in) throws IOException {
                 } else {
                     progress = in.readList(PhaseProgress::new);
                 }
-                if (in.getVersion().onOrAfter(Version.V_8_0_0)) {
+                if (in.getVersion().onOrAfter(Version.V_7_7_0)) {
                     memoryUsage = in.readOptionalWriteable(MemoryUsage::new);
                 } else {
                     memoryUsage = null;
@@ -315,7 +315,7 @@ public void writeTo(StreamOutput out) throws IOException {
                 } else {
                     out.writeList(progress);
                 }
-                if (out.getVersion().onOrAfter(Version.V_8_0_0)) {
+                if (out.getVersion().onOrAfter(Version.V_7_7_0)) {
                     out.writeOptionalWriteable(memoryUsage);
                 }
                 out.writeOptionalWriteable(node);

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

@@ -1,8 +1,3 @@
-setup:
-  - skip:
-      version: "all"
-      reason:  "Until backport of https://github.com/elastic/elasticsearch/pull/52778"
-
 ---
 "Get old outlier_detection job":
 

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

@@ -1,7 +1,4 @@
 setup:
-  - skip:
-      version: "all"
-      reason:  "Until backport of https://github.com/elastic/elasticsearch/pull/52778"
 
   - do:
       index:

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

@@ -1,8 +1,3 @@
-setup:
-  - skip:
-      version: "all"
-      reason:  "Until backport of https://github.com/elastic/elasticsearch/pull/52778"
-
 ---
 "Get old cluster outlier_detection job":