Improve time robustness by switching to chrony, trustworthy time sources and NTS

From chrony FAQ[1]:
"1.2. Should I prefer chrony over timesyncd if I do not need to run a
server?

Generally, yes.

systemd-timesyncd is a very simple NTP client included in the systemd
suite. It lacks almost all features of chrony and other advanced client
implementations listed on the comparison page. One of its main
limitations is that it cannot poll multiple servers at the same time and
detect servers having incorrect time (falsetickers in the NTP
terminology). It should be used only with trusted reliable servers,
ideally in local network.

Using timesyncd with pool.ntp.org is problematic. The pool is very
robust as a whole, but the individual servers run by volunteers cannot
be relied on. Occasionally, servers drift away or make a step to distant
past or future due to misconfiguration, problematic implementation, and
other bugs (e.g. in firmware of a GPS receiver). The pool monitoring
system detects such servers and quickly removes them from the pool DNS,
but clients like timesyncd cannot recover from that. They follow the
server as long as it claims to be synchronised. They need to be
restarted in order to get a new address from the pool DNS.

Note that the complexity of NTP and clock synchronisation is on the
client side. The amount of code in chrony specific to NTP server is very
small and it is disabled by default. If it was removed, it would not
significantly reduce the amount of memory or storage needed."

This commit fixes the issue by switching to a proper NTP client
(chrony), trustworthy time sources from Netnod and
Physikalisch-Technische Bundesanstalt which distributes the official
time for Sweden[2] and Germany[3] respectively, and finally NTS is used
to protect against MITM attacks.

Since most of our servers are in Germany or Finland (close to Sweden),
it makes sense to use these time sources as a low round-trip delay[4] is
preferred for NTP. For the few servers[5] we have outside Europe, the
root delay[4] will be higher than desired, but with the current use-case
for these servers, it should not be a problem.

[1] https://chrony-project.org/faq.html#_should_i_prefer_chrony_over_timesyncd_if_i_do_not_need_to_run_a_server
[2] https://www.netnod.se/swedish-distributed-time-service
[3] https://www.ptb.de/cms/en/ptb/fachabteilungen/abt4/fb-44/ag-442/dissemination-of-legal-time.html
[4] https://blog.meinbergglobal.com/2021/02/25/the-root-of-all-timing-understanding-root-delay-and-root-dispersion-in-ntp/
[5] {america,asia,sydney}.mirror.pkgbuild.com