playbook: switch to Chrony everywhere

Chrony is the default time daemon on Ubuntu and I'd like to keep it on the notepod because it « can perform usefully in an environment where access to the time reference is intermittent » and it « wakes up the CPU only when necessary, which is better for power saving » (https://chrony-project.org/comparison.html).

Overall, chrony looks better then ntpsec, even on the oldest distributions we support, since SLES switched to it in 2018. Note that the default on Debian is systemd-timesyncd.