Use github.token instead of missing MATZBOT_DEPENDABOT_MERGE_TOKEN secret

Author: k0kubun

.github/workflows/dependabot_automerge.yml

@@ -27,4 +27,4 @@ jobs:
         run: gh pr merge --auto --rebase "$PR_URL"
         env:
           PR_URL: ${{ github.event.pull_request.html_url }}
-          GH_TOKEN: ${{ secrets.MATZBOT_DEPENDABOT_MERGE_TOKEN }}
+          GH_TOKEN: ${{ github.token }}