mergify: automate the backport to 8.x when PRs targeting main (#5479)

* mergify: automate the backport to 8.x when PRs targeting main

* Update .mergify.yml

* Update .mergify.yml

Author: v1v

.mergify.yml

@@ -124,10 +124,11 @@ pull_request_rules:
           branches, such as:
           * `backport-v./d./d./d` is the label to automatically backport to the `8./d` branch. `/d` is the digit
 
-          **NOTE**: `backport-skip` has been added to this pull request.
+
+          `backport-v8.x` has been added to help with the transition to the new branch 8.x.
       label:
         add:
-          - backport-skip
+          - backport-v8.x
   - name: remove-backport label
     conditions:
       - label~=backport-v