Migrate CriticalConnection (deprecated) to KeepConfiguration (LP: #1896799)

[slyon]

Description

With systemd-v243 (which is the default since focal), the "CriticalConnection" attribute was
revamped into "KeepConfiguration" [1], addressing several issues, especially in high-availability
environents [2][3].

This PR is preparatory work for #409

If I understand the code correctly, it still uses "CriticalConnection" when setting "critical: true" in the netplan configuration.

• [1] https://github.com/systemd/systemd/pull/12511/files
• [2] systemd-networkd prunes floating virtual IPs for high-availability environments systemd/systemd#12050
• [3] https://chr4.org/blog/2019/01/21/make-keepalived-play-nicely-with-netplan-slash-systemd-network/

Closes: https://bugs.launchpad.net/netplan/+bug/1896799

Checklist

• Runs make check successfully.
• Retains 100% code coverage (make check-coverage).
• [] New/changed keys in YAML format are documented.
• [] (Optional) Adds example YAML for new feature.
• (Optional) Closes an open bug in Launchpad. LP#1896799

[slyon]

@chr4 Are you OK with getting this landed first before we continue with the extension of KeepConfiguration= as described in #409?

The extension requires some consensus on new Netplan YAML schema, so it takes a bit longer, but the few lines from this PR should already fix the issue, while providing a (limited) KeepConfiguration= setting.

Let me know what you think!

PS: I attributed the changes to your name.

[chr4]

No issues with taking the long road here, I agree that this is more consistent (and solves a few issues that came up in the discussion of #409).

PS: I attributed the changes to your name.

Thanks, much apprechiated!

[pajod]

They put it into Network section, though it still modifies DHCPv4 behaviour.
DHCP.CriticalConnection / DHCPv4.CriticalConnection => Network.KeepConfiguration
https://www.freedesktop.org/software/systemd/man/latest/systemd.network.html#KeepConfiguration=

[slyon]

Very good catch, thanks a lot! Fixed in 76e896a