[ML] Adjust BWC versions and unmute tests after backport (#56193)

... of #56118

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

@@ -127,19 +127,9 @@ public class Regression implements DataFrameAnalysis {
         boostedTreeParams = new BoostedTreeParams(in);
         predictionFieldName = in.readOptionalString();
         trainingPercent = in.readDouble();
-        if (in.getVersion().onOrAfter(Version.V_7_6_0)) {
-            randomizeSeed = in.readOptionalLong();
-        } else {
-            randomizeSeed = Randomness.get().nextLong();
-        }
-        if (in.getVersion().onOrAfter(Version.V_8_0_0)) {
-            lossFunction = in.readEnum(LossFunction.class);
-            lossFunctionParameter = in.readOptionalDouble();
-        } else {
-            // Prior to introducing the loss function setting only MSE was implemented
-            lossFunction = LossFunction.MSE;
-            lossFunctionParameter = null;
-        }
+        randomizeSeed = in.readOptionalLong();
+        lossFunction = in.readEnum(LossFunction.class);
+        lossFunctionParameter = in.readOptionalDouble();
     }
 
     public String getDependentVariable() {
@@ -181,13 +171,9 @@ public class Regression implements DataFrameAnalysis {
         boostedTreeParams.writeTo(out);
         out.writeOptionalString(predictionFieldName);
         out.writeDouble(trainingPercent);
-        if (out.getVersion().onOrAfter(Version.V_7_6_0)) {
-            out.writeOptionalLong(randomizeSeed);
-        }
-        if (out.getVersion().onOrAfter(Version.V_8_0_0)) {
-            out.writeEnum(lossFunction);
-            out.writeOptionalDouble(lossFunctionParameter);
-        }
+        out.writeOptionalLong(randomizeSeed);
+        out.writeEnum(lossFunction);
+        out.writeOptionalDouble(lossFunctionParameter);
     }
 
     @Override

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/dataframe/analyses/RegressionTests.java

@@ -63,8 +63,8 @@ public class RegressionTests extends AbstractBWCSerializationTestCase<Regression
             instance.getPredictionFieldName(),
             instance.getTrainingPercent(),
             instance.getRandomizeSeed(),
-            version.onOrAfter(Version.V_8_0_0) ? instance.getLossFunction() : null,
-            version.onOrAfter(Version.V_8_0_0) ? instance.getLossFunctionParameter() : null);
+            instance.getLossFunction(),
+            instance.getLossFunctionParameter());
     }
 
     @Override

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/56118"
-
 ---
 "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/56118"
 
   - 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/56118"
-
 ---
 "Get old cluster outlier_detection job":