[hashicorp/consul] Add deprecation notice (#2283)

Adds an official image deprecation notice/warning, and points end users to the verified publisher images at hashicorp/consul.

consul/deprecated.md

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+Upcoming in Consul 1.16, we will stop publishing official Dockerhub images and publish only our Verified Publisher images. Users of Docker images should pull from [hashicorp/consul](https://hub.docker.com/r/hashicorp/consul) instead of [consul](https://hub.docker.com/_/consul). Verified Publisher images can be found at https://hub.docker.com/r/hashicorp/consul.