The repository 'https://debian.cassandra.apache.org 311x Release' does not have a Release file.

The debian repo for 311 can't be used anymore; apt update fails with

Ign:10 https://debian.cassandra.apache.org 311x InRelease
Err:12 https://debian.cassandra.apache.org 311x Release
 server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt CRLfile: none
Reading package lists... Done  
E: The repository 'https://debian.cassandra.apache.org 311x Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.