The signing method to use when signing commits and tags. Valid values are openpgp (OpenPGP/GnuPG), ssh (SSH), and x509 (X.509 certificates).
openpgp
ssh
x509
null or one of "openpgp", "ssh", "x509"
"openpgp" for state version < 25.05, undefined for state version ≥ 25.05
