Upgrade old versions of Linux Mint

In the same vein as #21, several of our desktop machines are still running Linux Mint 17 and 18, which went out of LTS support in 2019 and 2021, respectively.

As far as I can tell from OCSInventory, the current candidates are:

• cerberus (a bit more complicated than the rest)
• porcupine
• pinball

While we're at it, it's probably worth upgrading the Mint 19 machines as well. Based on Ubuntu 18.04, the packages are starting to get a bit stale. However, Mint 21 should be coming out in the next months so those upgrades may well be best left until then.