Use different DCO action implementation

Switching from <https://github.com/tim-actions/dco> to <https://github.com/tisonkun/actions-dco>.
While both implementations look very similar there are some differences:

- The later implementation does not require use of additional action to pull PR commits because
it does not rely on `commits` being setup in `core` context.
- The later implementation is explicit about its LICENSE (AL2).

Signed-off-by: Lukáš Vlček <lukas.vlcek@aiven.io>

Author: lukas-vlcek

.github/workflows/dco.yml

@@ -10,12 +10,5 @@ jobs:
       pull-requests: read
 
     steps:
-      - name: Get PR Commits
-        id: 'get-pr-commits'
-        uses: tim-actions/get-pr-commits@v1.3.0
-        with:
-          token: ${{ secrets.GITHUB_TOKEN }}
       - name: DCO Check
-        uses: tim-actions/dco@v1.1.0
-        with:
-          commits: ${{ steps.get-pr-commits.outputs.commits }}
+        uses: tisonkun/actions-dco@v1.1