Push to instrumenta on release or create deprecation plan

[jpreese]

PR #411 updated the Circle config to push on release, but did not include the instrumenta project.

From @garethr's comment in #417 (comment), an announcement should be put together stating its official deprecation.

To resolve this, the Circle config should be updated to also push to instrumenta or formally announce that instrumenta is deprecated in favor of the openpolicyagent org (this gets my vote).

@garethr @Blokje5

[Blokje5]

I am in favor of deprecation as well. If we start sharing now that it will be deprecated, we can ensure people have time to update before the next release.

The open question for me is what channels we should use to share. We can leave a note on conftest.dev and on the OPA slack and hopefully that is enough notice.

[jpreese]

I feel as long as the DockerHub README had a blurb about the migration (e.g. https://hub.docker.com/_/opensuse), and we made a note of it in the 0.22.0 release notes on GitHub, that would cover enough for users to be able to easily find the new image.

FWIW there doesn't appear to be a paved path for this docker/hub-feedback#907

[Blokje5]

Updating the Dockerhub README will probably cover it. I also created #423 as I noted that the install documentation still referred to the instrumenta image. @garethr could you update the Dockerhub README of the instrumenta image?

[jpreese]

Updated our 0.22.0 release to include a note about using the openpolicyagent Docker image from DockerHub.