Update project license to GPL-3

[tsjensen]

As of today, the project license is not perfectly clear (see #97 and #95 (comment)). This task is to clean this up.

The intended license from the start of the project has always been GPL-2, which is mostly because at the time, this was the most recent version of GPL, and we didn't trust the FSF enough to opt in to the auto-upgrade. Maybe that's paranoid, but as they say, just because you're paranoid, doesn't mean they're not out to get you.

Now, while researching how to properly apply GPL-2 (as opposed to GPL-2+), we found that GPL-3 is really the better license for boxes, and we should upgrade to that. Richard Stallman explains the reasons why.

We expect that this change will have no impact on users.

[tsjensen]

Now we are properly on GPL-3.0-only for all source files.
Files in the misc/ folder are GPL-2.0-or-later, which we interpret as GPL-3, so we have compatibility.

Also, the libraries we depend upon have been checked for license compatibility.
The results are documented in a CycloneDX-compatible SBOM.