Buster no longer gets security updates

[JssDWt]

It looks like Umbrel OS is based on buster, but buster is not receiving security updates since June 30 2022.

Also, LND is planning to remove buster support for their builds soon: lightningnetwork/lnd#7944

[lukechilds]

The LND issue shouldn't affect us since it runs in a container. But yeah, we still need to bring the Pi umbrelOS up to date. We're currently in the process of completely rewriting the umbrelOS build system in a way that can be shared between Raspberry Pi and amd64 systems using Debian Bookworm as a base.