🚚 Add co-author information at the end of commit messages (#5852)

This PR adds co-author information at the end the mergify commit messages. GitHub parses lines containing `Co-Authored-By` at the end of the commit message and adds them as co-authors.

This should solve weblate getting all the recognition from translations and actual translators to co-author their translation PRs.

I've tried this in a forked repo but I wasn't able to test an actual co-author so probably we'll need some trial and error testing.

.mergify.yml

@@ -8,6 +8,11 @@ queue_rules:
     commit_message_template: |-
       {{ title }} (#{{number}})
       {{ body }}
+      {# Add co-author information at the end of the squash merge commit message #}
+      {% for co_author in co_authors | unique %}
+      Co-Authored-By: {{ co_author.name }} <{{ co_author.email }}>
+      {% endfor %}
+      {# `Co-Authored-By` lines must the be last ones for GitHub to recognize them #}
 
 pull_request_rules:
   - name: Automatic squash merge on approval
@@ -21,4 +26,4 @@ pull_request_rules:
       comment:
         message: Thank you for contributing! Your pull request is now going on the merge train (choo choo! Do not click update from main anymore, and be sure to [allow changes to be pushed to your fork](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/allowing-changes-to-a-pull-request-branch-created-from-a-fork)).
       queue:
-        name: default_queue
+        name: default_queue