xmpp: Shorten the default caps node to our main domain

We do own xmpp.rs, so let’s use that instead of gitlab.com.

xmpp/ChangeLog

@@ -26,6 +26,8 @@ XXXX-YY-ZZ [ RELEASER <admin@localhost> ]
       - Agent::wait_for_events now return Vec<Event> and sets inner tokio_xmpp Client
         auto-reconnect to true... It is still aware of Event::Disconnected but should
         no longer have weird hanging state or panic (!436)
+      - The default caps node has been shortened to https://xmpp.rs since we
+        own that domain. Users should change it in their application though.
     * CI:
       - Enforce changelog updates in Gitlab CI.