[ML] Unmute DFA BWC tests and fix versions after backport of #50914 (#50979)

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/dataframe/analyses/BoostedTreeParams.java

@@ -92,7 +92,7 @@ public class BoostedTreeParams implements ToXContentFragment, Writeable {
         eta = in.readOptionalDouble();
         maximumNumberTrees = in.readOptionalVInt();
         featureBagFraction = in.readOptionalDouble();
-        if (in.getVersion().onOrAfter(Version.V_8_0_0)) {
+        if (in.getVersion().onOrAfter(Version.V_7_6_0)) {
             numTopFeatureImportanceValues = in.readOptionalInt();
         } else {
             numTopFeatureImportanceValues = null;
@@ -106,7 +106,7 @@ public class BoostedTreeParams implements ToXContentFragment, Writeable {
         out.writeOptionalDouble(eta);
         out.writeOptionalVInt(maximumNumberTrees);
         out.writeOptionalDouble(featureBagFraction);
-        if (out.getVersion().onOrAfter(Version.V_8_0_0)) {
+        if (out.getVersion().onOrAfter(Version.V_7_6_0)) {
             out.writeOptionalInt(numTopFeatureImportanceValues);
         }
     }

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/50914"
-
 ---
 "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/50914"
 
   - 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/50914"
-
 ---
 "Get old cluster outlier_detection job":