fix: put annotations only within the index manifest (#659)

After digging into the source code of ArtifactHub, I believe
it doesn't make much sense to add ArtifactHub-specific
annotations within individual image manifests. What should
be changed is ArtifactHub, not this tool.

Since GitHub registry also expects annotations at the
index level. 'manifest' was removed. This is partially
reverting PR #652.

.github/workflows/build.yaml

@@ -66,7 +66,7 @@ jobs:
             io.artifacthub.package.readme-url=https://github.com/raw/favonia/cloudflare-ddns/main/build/SUMMARY.markdown
             io.artifacthub.package.maintainers=[{"name":"favonia","email":"favonia@email.com"}]
         env:
-          DOCKER_METADATA_ANNOTATIONS_LEVELS: manifest,index
+          DOCKER_METADATA_ANNOTATIONS_LEVELS: index
       - name: 🚀 Build and deploy minimal Docker images
         uses: docker/build-push-action@4a13e500e55cf31b7a5d59a38ab2040ab0f42f56 # v5.1.0
         id: build