Add clarification on the support policy for the docker machine executor to dockermachine.md

What does this MR do?

Adds clarification regarding the support policy for the Docker Machine executor. See related Slack thread.

Why was this MR needed?

There was a minor discrepancy in content between the language used on this docs page and the language in the deprecation notices page. This resulted in customers not understanding if they could in fact continue to use the Docker Machine executor in production environments.