upgrades: Detect and update to next release

When testing Buster -> Bullseye upgrade process, I found that unattended-upgrades would only upgrade for the current release. So I needed to run apt full-upgrade to completely upgrade. But this isn't ideal.

Find and document the correct way to upgrade from one release to the next.

Tasks:

• Implement distribution upgrades.
• Introduce a new flag for distribution upgrades.
• Make an announcement about option for distribution upgrades.
• tt-rss and bind also have configuration file prompts and should be locked during upgrade.
• Check if the upgrade process will finish successfully when spawned from FreedomBox daemon. Consider moving it to systemd service.
• Explore the possibility of using unattended-upgrades for the upgrade process.
• During a test with either TT-RSS or Bind installed, I found that freedombox package was removed during upgrade. Find a way to prevent this.
• Hold mumble-server during upgrade. Implement force upgrade SSL certificate and key set in /etc/mumble-server.ini.
• Avoid error when configuring grub-pc, when no device has been selected for installing grub (seen in Virtualbox).
• Update search engine list for searx to avoid uwsgi restarting issue.
• Current process holds freedombox package so it is not upgraded. Ensure it gets upgraded by end of process.
• Test dist upgrade for all apps.