fix: two bugs in cleanup.yml

The package name was missing the "ghcr.io/" prefix.
The value of `delete-only-untagged-versions` should've been `"true"` not `true`.

Author: ryboe

.github/workflows/cleanup.yml

@@ -14,7 +14,7 @@ jobs:
       - name: Delete untagged images
         uses: actions/delete-package-versions@v5
         with:
-          package-name: ${{ github.repository }}
+          package-name: ghcr.io/${{ github.repository }}
           package-type: container
-          delete-only-untagged-versions: true
+          delete-only-untagged-versions: "true"
           min-versions-to-keep: 0