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