How to fix apt update error NO_PUBKEY 78BD65473CB3BD13

If you encounter this error message during APT update:

The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 78BD65473CB3BD13

you need to import the key using

sudo apt-key adv --keyserver hkp://pool.sks-keyservers.net:80 --recv-keys 78BD65473CB3BD13