Use a non default port range in MockTransportService

We already use a per JVM port range in MockTransportService. Yet,
it's possible that if we are executing in the JVM with ordinal 0 that
other clusters reuse ports from the mock transport service and some tests
try to simulate disconnects etc. By using a non-defautl port range (starting at 10300)
we prevent internal test clusters from reusing any of the mock impls ports

Relates to #25301

test/framework/src/main/java/org/elasticsearch/test/transport/MockTransportService.java

@@ -106,7 +106,7 @@ public final class MockTransportService extends TransportService {
         // concurrent tests could claim port that another JVM just released and if that test tries to simulate a disconnect it might
         // be smart enough to re-connect depending on what is tested. To reduce the risk, since this is very hard to debug we use
         // a different default port range per JVM unless the incoming settings override it
-        int basePort = 9300 + (JVM_ORDINAL * 100);
+        int basePort = 10300 + (JVM_ORDINAL * 100); // use a non-default port otherwise some cluster in this JVM might reuse a port
         settings = Settings.builder().put(TcpTransport.PORT.getKey(), basePort + "-" + (basePort+100)).put(settings).build();
         NamedWriteableRegistry namedWriteableRegistry = new NamedWriteableRegistry(ClusterModule.getNamedWriteables());
         final Transport transport = new MockTcpTransport(settings, threadPool, BigArrays.NON_RECYCLING_INSTANCE,