Post-quantum security for SSH access on GitHub

This post details the introduction of a hybrid post-quantum key exchange algorithm (`sntrup761x25519-sha512`) for SSH connections to GitHub. It explains the 'store now, decrypt later' threat posed by quantum computers and how combining a new post-quantum algorithm with a classical one provides security against such attacks. It also outlines the rollout plan and provides instructions for users to check their SSH client's support for the new algorithm.