landscape-client uses deprecated apt-key

landscape-client still references apt-key, which is deprecated upstream (https://blog.jak-linux.org/2021/06/20/migrating-away-apt-key/):

$ grep -r apt-key .
./dev/landscape-client-vm:apt-key add /root/ppa-key
./landscape/client/manager/aptsources.py: self._run_process("/usr/bin/apt-key", ["add", path]))
./landscape/client/manager/tests/test_aptsources.py: C{AptSources} runs a process with apt-key for every keys in the
./landscape/client/manager/tests/test_aptsources.py: self.assertEqual("/usr/bin/apt-key", command)
./landscape/client/manager/tests/test_aptsources.py: The files created to be imported by C{apt-key} are removed after the
./landscape/client/manager/tests/test_aptsources.py: The files created to be imported by C{apt-key} are removed after the
./landscape/client/manager/tests/test_aptsources.py: If the C{apt-key} command failed for some reasons, the output of the
./landscape/client/manager/tests/test_aptsources.py: If the C{apt-key} fails with a signal, the output of the command is
./landscape/client/manager/tests/test_aptsources.py: If the C{apt-key} command failed for some reasons, the current
$

apt-key will eventually go away, and landscape-client needs to adapt.