added update branch before auto-merge (#429)

Updated `auto-merge-bot` to version
[`1.1.0`](https://github.com/paritytech/auto-merge-bot/releases/tag/v1.1.0)
which has the option to update a branch to be up-to-date before merging.

- [x] Does not require a CHANGELOG entry

.github/workflows/auto-merge.yml

@@ -23,8 +23,9 @@ jobs:
           app-id: ${{ secrets.MERGE_APP_ID }}
           private-key: ${{ secrets.MERGE_APP_KEY }}
       - name: Set auto merge
-        uses: paritytech/auto-merge-bot@v1.0.1
+        uses: paritytech/auto-merge-bot@v1.1.0
         with:
           GITHUB_TOKEN: ${{ steps.merge_token.outputs.token }}
           MERGE_METHOD: "SQUASH"
           ALLOWLIST: ${{ steps.fellows.outputs.github-handles }}
+          UPDATE_BEFORE_MERGE: true