Use resolvectl instead of systemd-resolve

[mbiebl]

The latter is a deprecated compat symlink.

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=979267

[mbiebl]

From the downstream bug report:

Hi,

systemd-resolve has been replaced by the resolvectl tool in systemd v239
(i.e. is available since buster). This is from the systemd release
notes:

* The systemd-resolve tool has been renamed to resolvectl (it also
  remains available under the old name, for compatibility), and its
  interface is now verb-based, similar in style to the other <xyz>ctl
  tools, such as systemctl or loginctl.

systemd-resolve is nowadays merely a symlink pointing at resolvectl and
we'd like to get rid of this compat symlink at some point.

Your package uses the old name like this:

$ grep -E "systemd-resolve\b" -R
run-tests:function systemd-resolve {
run-tests:  _log "systemd-resolve called with: ${@}"
README.md:`openvpn`, which will use the `update-systemd-resolve.conf` file instead:
README.md:$ systemd-resolve eu-central-1.console.aws.amazon.com
README.md:$ systemd-resolve eu-central-1.console.aws.amazon.com
update-systemd-resolved:    systemd-resolve --flush-caches

Attached is a patch which uses resolvectl instead.
Please review and consider applying it in your next upload.
It would be great if you can make this upload before the bullseye
release, so we can safely drop the symlink in bullseye+1.