ci: trigger compatibility workflow on the feature branch

Add a push trigger for test/cross-version-chat-compat so the workflow runs on
CI before merge (workflow_dispatch only works once the file is on the default
branch). To be removed before merge.

Author: Egor Rachkovskii

.github/workflows/functional-tests-compatibility.yml

@@ -1,6 +1,9 @@
 name: Functional Tests - Compatibility
 
 on:
+  push:
+    branches:
+      - test/cross-version-chat-compat
   workflow_dispatch:
     inputs:
       peer_refs: