Make automatic backporting PRs take the title from the original one

Up until now, the title of the backporting PRs was the name of the merge
commit, which is not very readable. This commit builds the title from
the original PR title and the target branch, e.g.:

    [v4.2] Fix some bug

This is now possible thanks to upstream's recent changes. See
sorenlouv/backport#455 for details.

.backportrc.json

@@ -0,0 +1,3 @@
+{
+  "prTitle": "[{{targetBranch}}] {{sourcePullRequest.title}}"
+}

.github/workflows/backport.yml

@@ -18,7 +18,7 @@ jobs:
       )
     steps:
       - name: Backport Action
-        uses: sqren/backport-github-action@v8.9.3
+        uses: sqren/backport-github-action@v9.3.0
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
           auto_backport_label_prefix: backport-