Add deprecation notice to Vault (#2291)

See also https://github.com/docker-library/docs/pull/2283

vault/deprecated.md

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+Upcoming in Vault 1.14, we will stop publishing official Dockerhub images and publish only our Verified Publisher images. Users of Docker images should pull from [hashicorp/vault](https://hub.docker.com/r/hashicorp/vault) instead of [vault](https://hub.docker.com/_/vault). Verified Publisher images can be found at https://hub.docker.com/r/hashicorp/vault.