mergify: change description regarding the backport process (#11402)

Author: v1v

.mergify.yml

@@ -170,8 +170,8 @@ pull_request_rules:
           This pull request does not have a backport label. Could you fix it @{{author}}? 🙏
           To fixup this pull request, you need to add the backport labels for the needed
           branches, such as:
-          * `backport-7.x` is the label to automatically backport to the `7.x` branch.
-          * `backport-7./d` is the label to automatically backport to the `7./d` branch. `/d` is the digit
+          * `backport-7.17` is the label to automatically backport to the 7.17 branch.
+          * `backport-8./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.
       label: