Remove WireGuard PPA

Given that the PPA is no longer available or necessary, and that it causes an error, let's remove it.

This should also solve incompatibility problems with Debian, which doesn't play nicely with PPAs.

Both OSes can install WG directly from their own repositories now.

Remaining tasks:

1. Remove PPA from install tasks.
2. Test.
3. Update min. OS requirements for Ubuntu: 16.04+
4. Publish new version of the collection.