CI: Force changelog updates

Changelog updates are now required in MRs by default. If there is a
match for "\<skip.\?changelog\>" in the commit message, this will be
ignored.

$CI_COMMIT_MESSAGE mangles newlines so it's not possible to require the
string to be on its own line, that means it would also trigger within a
wall of text, which makes it less obvious.

Also, I wasn't able to find a CI variable which has the ref of the
branch the MR is set against, to build a tree-ish than spans over the
whole MR and not just HEAD.

Signed-off-by: Maxime “pep” Buquet <pep@bouah.net>

xmpp/ChangeLog

@@ -24,6 +24,8 @@ 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)
+    * CI:
+      - Enforce changelog updates in Gitlab CI.
 
 Version 0.6.0:
 2024-07-27 [ Maxime “pep” Buquet <pep@bouah.net> ]