ci: revert to auto GITEA_TOKEN for registry auth

Gitea 1.26 (PR #36173) honors permissions.packages: write on the
auto-provided GITEA_TOKEN, so the PAT workaround is no longer needed.
You can delete the REGISTRY_TOKEN org secret.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitea/workflows/rollback.yml

@@ -78,8 +78,8 @@ jobs:
         uses: docker/login-action@b45d80f862d83dbcd57f89517bcf500b2ab88fb2 # v4.0.0
         with:
           registry: git.farh.net
-          username: gitea-admin
-          password: ${{ secrets.REGISTRY_TOKEN }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITEA_TOKEN }}
 
       - name: Verify Docker image tag exists
         run: docker buildx imagetools inspect "git.farh.net/farhoodlabs/trebuchet:${{ steps.target.outputs.version }}"