Automatically label PRs to main with backport-v8.x (elastic#40718)

* Automatically label PRs to `main` with `backport-8.x`

When the `main` branch switches over to `9.0`, we will want most PRs that are merged into
`main` to also be backported to `8.x`. This PR makes this process easier by automatically
adding the `backport-8.x` label to all PRs targeting the `main` branch.

The exceptions will be PRs for changes that should _only_ be present in `9.0`.  For such PRs,
authors should simply remove the `backport-8.x` label from their PR.

* Fix typo in comment

* Add missing v prefix to label for consistent with backport PR rule

.mergify.yml

@@ -147,6 +147,11 @@ 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.
+      label:
+        add:
+          - backport-v8.x
   - name: notify the backport has not been merged yet
     conditions:
       - -merged