Status Code in ReverseProxyTestCase should be 504

There is a CI failure on undertow upgrade PR: https://github.com/wildfly/wildfly-core/pull/4872

The test failure is:

org.jboss.as.test.integration.web.reverseproxy.ReverseProxyTestCase.testReverseProxyMaxRequestTime: java.lang.AssertionError: Service Unaviable expected because max-request-time is set to 10ms expected:<503> but was:<504>
	at org.jboss.as.test.integration.web.reverseproxy.ReverseProxyTestCase.testReverseProxyMaxRequestTime(ReverseProxyTestCase.java:251)

With https://issues.redhat.com/browse/UNDERTOW-1459 fixed, when a server that's acting as a gateway or proxy and did not receive a timely response from the upstream server should return status code 504 Gateway Timeout

It is recommended to fix the test failure in 2 steps:

• Step 1: Update the test case to accept either 503 or 504 to pass the CIs in wildfly-core
• Step 2: Update the test case to accept 504 only after the corresponding wildfly-core version gets integrated into WildFly full

So there will be 2 PRs as well to fix this completely.