The one-liner that fixes this is the following:
sudo apt-key adv --recv-keys --keyserver ha.pool.sks-keyservers.net 5072E1F5
Which basically fetches the new key and installs it into the keyring.
A bit more detail:
You can find the expired key by executing the following command:
LANG=C apt-key list | grep expired
Which outputs something like this:
pub 1024D/5072E1F5 2003-02-03 [expired: 2019-02-17]
5072E1F5 is the key ID, which we used in the command above.
Another one-liner that's recommended in the bug report below is this one:
sudo apt-key adv --recv-keys --keyserver pgp.mit.edu 5072E1F5
But I couldn't get it to work.
More reading from when this happened the last time.