Remove redundant category in generated release notes

The release notes generator only outputs an issue in the first
matching category. This means the 'Deprecation notices' category is
redundant and will never be produced.

Instead, combine 'Breaking changes' and 'Deprecation notices' into a
single category.

.github/release.yml

@@ -3,10 +3,7 @@ changelog:
     labels:
       - release-note-none
   categories:
-    - title: 🚨 Breaking changes
-      labels:
-        - release-note-action-required
-    - title: 🚨 Deprecation notices
+    - title: 🚨 Breaking changes / deprecation notices
       labels:
         - release-note-action-required
     - title: ✨ Features