Bonjour je n'arrive pas à mettre à jour et ceci s'affiche à chaque fois

root@vps593101:/# sudo apt-get update
Ign:1 http://ftp.debian.org/debian jessie-backports InRelease
Hit:2 http://ftp.debian.org/debian stretch-backports InRelease
Ign:3 http://deb.debian.org/debian stretch InRelease
Hit:4 http://security.debian.org stretch/updates InRelease
Hit:5 http://deb.debian.org/debian stretch-updates InRelease
Err:6 http://ftp.debian.org/debian jessie-backports Release
  404  Not Found [IP: 130.89.148.12 80]
Hit:7 http://deb.debian.org/debian stretch Release
Get:8 https://packages.sury.org/php stretch InRelease [6,760 B]
Hit:9 https://download.docker.com/linux/debian stretch InRelease
Err:8 https://packages.sury.org/php stretch InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743
Hit:11 https://packages.gitlab.com/gitlab/gitlab-ce/debian stretch InRelease
Hit:12 https://packages.gitlab.com/gitlab/gitlab-ee/debian stretch InRelease
Reading package lists... Done
E: The repository 'http://ftp.debian.org/debian jessie-backports Release' does no longer 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: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://packages.sury.org/php stretch InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B188E2B695BD4743

Bonjour,

Probléme GPG, lis la derniére ligne (20e)