ggcr: referrers Content-Type should indicate it is an OCI index

[sudo-bmitch]

Describe the bug

The referrers response should be an OCI index, but the Content-Type header says it's text/plain.

To Reproduce

Run the server with referrers enabled, push content with referrers, and query with curl -i to see the headers.

Expected behavior

The header should be Content-Type: application/vnd.oci.image.index.v1+json.

Additional context

opencontainers/distribution-spec#452