Upgrade everything that still upgrades. Add these to your /etc/apt/sources.list
:
deb [trusted=yes] http://archive.debian.org/debian jessie main
deb [trusted=yes] http://archive.debian.org/debian-security jessie/updates main
Optionally, you can comment out the old entries you may already have there.
Run apt-get update && apt-get upgrade
Chances are, if you try to connect with what you got, Github will complain:
ERROR: You're using an RSA key with SHA-1, which is no longer allowed. Please use a newer client or a different key type. Please see https://github.blog/2021-09-01-improving-git-protocol-security-github/ for more information.
fatal: Could not read from remote repository.
Please make sure you have the correct access rights and the repository exists.
Generate a new key that Github will accept:
ssh-keygen -t ecdsa -b 521 -m PEM
Copy your key from cat ~/.ssh/id_ecdsa.pub
and add it to https://github.com/settings/keys
Sources: