[ML][TEST] Use java 11 valid time format in DataDescriptionTests (#31817)

It seems that java 11 tightened some validations with regard to
time formats. The random instance creator was setting an odd
time format to the data description which is invalid when run
with java 11. This commit changes it to a valid format.

Author: dimitris-athanasiou

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/job/config/DataDescriptionTests.java

@@ -14,7 +14,6 @@
 import org.elasticsearch.common.xcontent.XContentParser;
 import org.elasticsearch.common.xcontent.json.JsonXContent;
 import org.elasticsearch.test.AbstractSerializingTestCase;
-import org.elasticsearch.xpack.core.ml.job.config.DataDescription;
 import org.elasticsearch.xpack.core.ml.job.config.DataDescription.DataFormat;
 import org.elasticsearch.xpack.core.ml.job.messages.Messages;
 
@@ -255,7 +254,7 @@ protected DataDescription createTestInstance() {
             } else if (randomBoolean()) {
                 format = DataDescription.EPOCH_MS;
             } else {
-                format = "yyy.MM.dd G 'at' HH:mm:ss z";
+                format = "yyyy-MM-dd HH:mm:ss.SSS";
             }
             dataDescription.setTimeFormat(format);
         }