Even when the system resolver is configured to something else that
systemd-resolved, it still is a good idea to keep systemd-resolved up to
date. If not anything else, it does a good job at doing per-interface
resolving for connectivity checks.

If for whatever reasons don't want NetworkManager to push the DNS data
it discovers to systemd-resolved, the functionality can be disabled
with:

  [main]
  systemd-resolved=false