An apt repository has changed its key

[Bob Bernstein <poobah@ruptured-duck.com>]

At least, that's my diagnosis.

Here's the pertinent line from an entry in my 
/etc/apt/sources.list.d/:

deb http://ppa.launchpad.net/wireguard/wireguard/ubuntu focal main

Here's the output of 'apt-get update':

--snip--

0% [Working] 0% [Connecting to debian.map.fastlydns.net (2a04:4e4                                                    Err:1 http://ppa.launchpad.net/wireguard/wireguard/ubuntu focal InRelease
  403  Forbidden [IP: 91.189.95.83 80]
0% [Connecting to debian.map.fastlydns.net (2a04:4e4                                                    Hit:2 http://security.debian.org buster/updates InRelease
                                                    0% [Waiting for headers]                        Hit:3 http://deb.debian.org/debian buster InRelease
[...]

E: Failed to fetch http://ppa.launchpad.net/wireguard/wireguard/ubuntu/dists/focal/InRelease  403  Forbidden [IP: 91.189.95.83 80]
E: The repository 'http://ppa.launchpad.net/wireguard/wireguard/ubuntu focal InRelease' is no longer signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

--snip--

That apt repo line, above, has worked for months. Iirc, this is 
not my first rodeo with this problem. I forget how I remedied 
it.

I believe apt provides a nice solution for this, that will allow 
me to get the new key onto my machine?

Thank you.

--
A person of great honour in Ireland (who was pleased to
stoop so low as to look into my mind) used to tell me that
my mind was like a conjured spirit, that would do mischief
if I did not give it employment.
                                Jonathan Swift