Deprecate Proxy pull-through cache

https://gitlab.com/gitlab-org/container-registry/-/blob/master/docs/configuration.md#proxy

This section allows the registry to be set up as a local mirror to an upstream repository. This use case is less useful in the context of a registry for a deployment of GitLab.

Users who desire this may use the upstream registry implementation.

UPDATE: Based on a recent survey of Admin, we learned that 26% of you are using this feature. So, we will not deprecate this feature and continue to support it.