Update beats security docs to remove deprecated roles

[dedemorton]

Some Kibana roles were deprecated in 7.6:

Starting in 7.6, the kibana_user role has been deprecated in favor of a properly named kibana_admin role; and the kibana_dashboard_only_user has been deprecated as privileges can be assigned to individual features using Kibana Privileges. These deprecated roles will be surfaced to end-users starting in 7.7.

[dedemorton]

Also when you have this file open, make sure it's clear what we mean by "This section assumes that you’ve pre-loaded dependencies."

Essentially, users need to enable modules (if relevant) and run the setup.