I am having a similar issue, but --allow-releaseinfo-change does not let me finish the update.
@pihole:~ $ sudo apt-get update --allow-releaseinfo-change
Ign:1 https://deb.nodesource.com/node_14.x buster InRelease
Hit:2 http://archive.raspberrypi.org/debian buster InRelease
Hit:3 http://raspbian.raspberrypi.org/raspbian buster InRelease
Err:4 https://deb.nodesource.com/node_14.x buster Release
Certificate verification failed: The certificate is NOT trusted. The certificate chain uses expired certificate. Could not handshake: Error in the certificate verification. [IP: 104.123.154.145 443]
Reading package lists... Done
E: The repository 'https://deb.nodesource.com/node_14.x buster Release' no longer has 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.
Upon further investigation, the commands at this link were able to clear up the certificate problem: Link
Specifically, apt-get install -y ca-certificates libgnutls30 cleared it up and I was able to update FTL afterwards.