Add test to ensure backports from 9.1 are done correctly

server/src/test/java/org/elasticsearch/TransportVersionTests.java

@@ -27,6 +27,17 @@
 
 public class TransportVersionTests extends ESTestCase {
 
+    /**
+     * This test is specific for V9, to ensure that transport versions are backported correctly.
+     */
+    public void testMaximumAllowedTransportVersion() {
+        assertThat(
+            TransportVersions.LATEST_DEFINED.onOrBefore(TransportVersions.ELASTICSEARCH_9_0)
+                || TransportVersions.LATEST_DEFINED.isPatchFrom(TransportVersions.ELASTICSEARCH_9_0),
+            is(true)
+        );
+    }
+
     public void testVersionComparison() {
         TransportVersion V_8_2_0 = TransportVersions.V_8_2_0;
         TransportVersion V_8_16_0 = TransportVersions.V_8_16_0;