Update metrics dashboard section in environments docs and add deprecation notice
Problem to solve
The Environment docs have a section for metrics dashboards. The screenshot is out of date. Additionally, there should be a section relating to the removal/deprecation of the dashboard in 16.0.
See https://docs.gitlab.com/ee/ci/environments#monitor-environments for the relevant section.
Proposal
On https://docs.gitlab.com/ee/ci/environments/#monitor-environments
- Update or remove the screenshot.
- Add a deprecation warning.