mergify: add backport-v8.x always for main (#40741)

.mergify.yml

@@ -147,8 +147,16 @@ pull_request_rules:
           To fixup this pull request, you need to add the backport labels for the needed
           branches, such as:
           * `backport-v8./d.0` is the label to automatically backport to the `8./d` branch. `/d` is the digit
-
-          **NOTE**: `backport-v8.x` has been added to help with the transition to the new branch 8.x.
+  - name: add backport-v8.x label for main only
+    conditions:
+      - -label~=^backport-v8.x
+      - base=main
+      - -merged
+      - -closed
+    actions:
+      comment:
+        message: |
+          `backport-v8.x` has been added to help with the transition to the new branch `8.x`.
       label:
         add:
           - backport-v8.x