Docker Distribution v2.7.0 shipped with OCI support, but our container
registry client was not updated to handle the manifest format in the
HTTP `Accept` header.  As a result, API calls to retrieve a manifest
would return with an error, "OCI manifest found, but accept header does
not support OCI manifests". This would result in blank fields in the
container registry page and prevent tags from being deleted.

To fix this, we just need to add
`application/vnd.oci.image.manifest.v1+json` to the `Accept` header and
configure Faraday to parse the response as JSON. The response structure
is the same as the standard Docker Distribution V2 manifest.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58685

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/12877

There are some redundancies in the validation steps, and that is to
preserve current error messages behavior

Also few specs have to be changed in order to fix madness in validation
logic.

This reverts commit cb10b725c8929b8b4460f89c9d96c773af39ba6b.

This reverts commit 13e37a3e.

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

Also removes the note from the development/testing.md guide