platform property in OCI manifest list spec is optional but Quay makes it required

As per https://github.com/opencontainers/image-spec/blob/main/image-index.md?plain=1#L50 the platform property in the manifests block of an OCI index is optional. While the use of this structure for multi-arch images is prevalent, the specification calls it out as optional as OCI indexes are not limited to the use case of shipping the same containerized software for different compute architecture.

Expected behavior: Quay accepts OCI index manifests without values for `platform` in the `manifests` array.

Actual behavior: Quay rejects OCI index manifests without values for `platform` in the `manifests` array.