Github suffered a security incident that exposed their private SSH key, and they rotated it last Friday.
More info here: https://www.bleepingcomputer.com/news/security/githubcom-rotates-its-exposed-private-ssh-key/
The change caused the host key to be regenerated based on the new private key. Since you have a known_hosts entry for github that doesn't match the hostkey the server is presenting you now, you see this error to warn you that you might not be connecting to the system you think you are.
as to how to clean up the knownhosts issue, it depends on your OS/SSH client. for windows/openSSH see here: Where is the known_hosts file for OpenSSH for Windows?
for linux see here: https://www.howtouselinux.com/post/ssh-known_hosts-file
just delete the existing entry for github, and on next connect, accept the new host key to stop the confirmation prompts.