The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 46B5721DBBD2996A Err:10 https://ftp.postgresql.org/pub/pgadmin/pgadmin4/apt/focal pgadmin4 Release Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate. Could not handshake: Error in the certificate verification. [IP: 87.238.57.227 443]
But guessing somebody from the maintainers might need to renew the certificate or something?
Sounds like you need to update your system; Let's Encrypt had a root certificate expire at the end of September which has been causing problems for people who haven't updated their OS for a while.