Fix "relay-port" XSD type in spring-websocket.xsd

Prior to this commit, the `relay-port` attribute of the
`<websocket:stomp-broker-relay />` tag was of type `xsd:int`.
This prevents developers from using `PropertyPlaceholderConfigurer`,
even though this configuration key is a good candidate for such use
(this value depends on prod/staging/etc environment).

This commit changes that type to `xsd:string`.

Issue: SPR-11537

spring-websocket/src/main/resources/org/springframework/web/socket/config/spring-websocket-4.0.xsd

@@ -236,7 +236,7 @@
                 ]]></xsd:documentation>
             </xsd:annotation>
         </xsd:attribute>
-        <xsd:attribute name="relay-port" type="xsd:int">
+        <xsd:attribute name="relay-port" type="xsd:string">
             <xsd:annotation>
                 <xsd:documentation source="java:org.springframework.messaging.simp.stomp.StompBrokerRelayMessageHandler"><![CDATA[
 	The STOMP message broker port.