We’ve been signing with a new (2021) key since a while: https://www.theforeman.org/static/keys/5B7C3E5A735BCB4D615829DC0BDDA991FD7AAC8A.pub
Seems you don’t have it imported.
It’s also part of the keyring we publish at https://deb.theforeman.org/foreman.asc
See also the release notes for 3.0 where we started to use that key: Foreman :: Manual