How did I fix «Err http://repo.mysql.com/apt/debian jessie InRelease» / «The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8C718D3B5072E1F5» on Debian10 (Buster)?

no_pubkey, gpg, apt, debian-10-buster, debian, public-key-certificate, public-key-cryptography, mysql