Document how to disable and deprecate TLS 1.1

We plan to deprecate TLS 1.1 in 12.0 (#3875 (closed)). To prepare for that, we should:

1. Document how to disable TLS 1.1 where it is enabled (NGINX and Postgres) in our 11.x notes: https://docs.gitlab.com/omnibus/update/gitlab_11_changes.html#11-4
2. Add a deprecation note to the %11.6 blog post, along with instructions on how to disable it today if you would like.