Upload to Debian main

In order to be able to call apt install briar-desktop, we need to bring Briar Desktop into Debian's official repositories, specifically its main flavor and not contrib or non-free.

Being in Debian would mean that virtually all its descendants like Ubuntu and elementaryOS will also contain this package.

However, getting briar-desktop into Debian is potentially blocked by many issues. This is an epic issue to investigate and manage those issues:

• Kotlin is not yet in Debian Seems like it's now available in sid 🎉
• Briar most likely isn't allowed to ship its own tor binary, especially with focus on descendants like Tails. See briar#2095
• All packages must be build offline, therefore we can't download gradle dependencies while building
• Open question: do we have to create a new package for each Gradle dependency?

In case we want to do our own apt repository for the time being, keep in mind that there's a draft in https://code.briarproject.org/briar/briar-apt-repo/-/tree/main.

Sub-issues:

• #50