chore(mergifyio): use consistent conflict label (#4178)

Mergifyio applies the "conflicts" label whereas we apply the "conflict" label.

Example: #4138

Author: Kyle-Verhoog

.mergify.yml

@@ -42,11 +42,11 @@ pull_request_rules:
         message: "@{{author}} this pull request is now in conflict 😩"
       label:
         add:
-          - conflict
+          - conflicts
   - name: remove conflict label if not needed
     conditions:
       - -conflict
     actions:
       label:
         remove:
-          - conflict
+          - conflicts