[ML] Fix casting in RegressionIT test (elastic#136743) (elastic#137815)

(cherry picked from commit d8854e5)

# Conflicts:
#	muted-tests.yml

Author: davidkyle

x-pack/plugin/ml/qa/native-multi-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/ml/integration/RegressionIT.java

@@ -630,8 +630,8 @@ public void testAliasFields() throws Exception {
                 assertThat(resultsObject.containsKey(predictionField), is(true));
                 assertThat(resultsObject.containsKey("is_training"), is(true));
 
-                int featureValue = (int) destDoc.get("field_1");
-                double predictionValue = (double) resultsObject.get(predictionField);
+                int featureValue = ((Number) destDoc.get("field_1")).intValue();
+                double predictionValue = ((Number) resultsObject.get(predictionField)).doubleValue();
                 predictionErrorSum += Math.abs(predictionValue - 2 * featureValue);
             }
             // We assert on the mean prediction error in order to reduce the probability