WireGuard is included in Ubuntu by default now

[zx2c4]

WireGuard is now in Ubuntu 20.04, 18.04, and 16.04 in their default
kernels, and all other Ubuntus are EOL'd by Canonical. [1] has details
on replacing the PPA with Canonical's builtin repos, which this commit
implements.

[1] https://lists.zx2c4.com/pipermail/wireguard/2020-August/005737.html

---

I haven't tested this, so please test before merging. Also, everything usually works better after an apt update && apt upgrade && reboot as it usually takes Canonical a while to make Ubuntu work after the initial packages they release onto the ISO.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[davidemyers]

Jason, thanks for your work on WireGuard! But this PR looks redundant to #1855.

[zx2c4]

Oh, excellent, thanks!