Add generic support for all registries

[mseiwald]

• with this change, docker-mirror will support all docker registries that are supported by go-containerregistry
• I left the "special" support for dockerhub in-place to be backwards-compatible and because the generic way does not support getting the "last modified" date (which was already the case for the ghcr and k8s registries).
• I removed the "special" support for quay.io since it actually did not work. There was a timestamp parsing error so I removed it altogether. This only affect the ability to sort the tags by timestamp but as already said this did not work anyway when I tested it.
• Fixes support ghcr.io  #128