[CI, Docs] Update post-release notification instructions
[CI, Docs] Update post-release notification instructions

There is automation that posts announcements to the "Chef Release
Announcements" Discourse channel to a Chef-internal Slack channel, so
we should post there.

Additionally, a "merge freeze" is no longer necessary with our new
pipeline.

Signed-off-by: Christopher Maier <b4598f17a90ef80817bae8ad6c37249740caafb4@chef.io>

rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat