release upgrade broke networking

[Simon Harwood]

After upgrading one of my Ubuntu Server 18.04 LTS to 20.04.5 LTS systems, the network access was effectively destroyed.
I had upgraded using the ESXi virtual console and found that the interfaces file was still present but ignored. Instead, netplan had been installed with no configuration files and no information about migrating interfaces to the netplan yaml files - either assisted or how to do it manually.
The installation is effectively a headless server, and so instructions for the desktop based Network Manager system are not relevant.
With the help of several conflicting tutorials, I managed to mangle data from my interfaces file into a yaml file, resulting in networking that works some of the time. (One of the conflicts is where some tutorials say to use dhcp[46]: yes/no and others use dhcp[46]: true/false.)
The worst problem is that it will not communicate with my DNS server meaning that apt/apt-get cannot be used to revert to ifupdown. I can persuade it to accept the static IPv4 and IPv6 addresses that it should have but that doesn't make it able to communicate reliably, as it did previously.

[Lukas Märdian (slyon)]

I think this is a valid issue. There are some tools to provide help with migration, but especially the documentation around it needs to be improved.

$ apt install netplan.io
$ systemctl unmask systemd-networkd.service
$ systemctl unmask systemd-resolved.service
$ ENABLE_TEST_COMMANDS=1 netplan migrate
$ netplan try
$ reboot
$ apt purge ifupdown resolvconf
$ ln -sf /run/systemd/resolve/stub-resolv.conf /etc/resolv.conf