未署名のDebian Jessieリリースエラー[閉じる]

を使用してシステムを更新しようとしていますが、更新しようとsudo apt updateすると、次の結果が表示されます。

Hit:1 https://dl.winehq.org/wine-builds/ubuntu focal InRelease
Hit:2 https://deb.debian.org/debian buster InRelease                            
Hit:3 https://deb.debian.org/debian-security buster/updates InRelease           
Ign:4 http://ppa.launchpad.net/cybermax-dexter/sdl2-backport/ubuntu impish InRelease
Ign:6 https://storage.googleapis.com/cros-packages/91 buster InRelease          
Hit:7 https://storage.googleapis.com/cros-packages/91 buster Release            
Err:8 http://ppa.launchpad.net/cybermax-dexter/sdl2-backport/ubuntu impish Release
  404  Not Found [IP: 91.189.95.85 80]
Ign:5 http://cdn-fastly.deb.debian.org/debian jessie InRelease                  
Hit:10 http://deb.i2p2.no unstable InRelease                                    
Get:9 http://cdn-fastly.deb.debian.org/debian jessie Release [77.3 kB]
Get:11 http://cdn-fastly.deb.debian.org/debian jessie Release.gpg [1,652 B]
Ign:11 http://cdn-fastly.deb.debian.org/debian jessie Release.gpg
Reading package lists... Done
E: The repository 'http://ppa.launchpad.net/cybermax-dexter/sdl2-backport/ubuntu impish 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.
W: GPG error: http://cdn-fastly.deb.debian.org/debian jessie Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 7638D0442B90D010 NO_PUBKEY CBF8D6FD518E17E1
E: The repository 'http://http.debian.net/debian jessie Release' is not signed.
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.

この問題を解決する方法がわからないので、助けていただきありがとうございます。