Release v13.16.0

This MR links up with gitlab!53097 (merged).

In CHANGELOG:

• RFC: Simple built-in SSH server !394 (merged)
• Remove the session duration information from the output of 2fa_verify command !445 (merged)

Not in CHANGELOG:

• Remove default branch statement in docs !443 (merged)
• Add gitlab-sshd to .gitignore !444 (merged)