How to fix apt update EXPKEYSIG 1F3045A5DF7587C3

Problem:

When running sudo apt update you see the following error message:

Err:19 https://josm.openstreetmap.de/apt focal InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 130A439C78FC0F87
Reading package lists... Done
W: GPG error: https://josm.openstreetmap.de/apt focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 130A439C78FC0F87

Solution

Add the JOSM key using

wget -q https://josm.openstreetmap.de/josm-apt.key -O- | sudo apt-key add -

After that, the error should be fixed and you can run

sudo apt update

again.