System upgrade

[lenemter]

ToDo:

• Update repository files
• Install systemd-resolved
• Do the upgrade
• sudo touch /etc/NetworkManager/conf.d/10-globally-managed-devices.conf
• Pull io.elementary.settings
• ? Install Flathub if it's not present

[leolost2605]

Hey haven't looked much at this PR just wanted to share some things since I've also looked into this in the past (https://github.com/leolost2605/upgrader although it's really ugly and doesn't work and shouldn't be taken as reference 😅).
For your minimized comment you can use the DBus API of apt. Also in general a good(?) reference is the Zorin OS upgrader which is also written in Vala and is pretty much the same use case that we have, i.e. Ubuntu only upgrades. I haven't found their git repo but you can download the source as a tar at https://launchpad.net/~zorinos/+archive/ubuntu/stable/+sourcepub/16233056/+listing-archive-extra