Release v13.18.0

In CHANGELOG:

- Fix thread-safety issues in gitlab-shell !463
- gitlab-sshd: Support the PROXY protocol !461
- sshd: Recover from per-session and per-connection panics !464

Not in CHANGELOG:

- Use Docker 20.10.3 instead of 19.03.0 on build !456
- Compile against go 1.14-1.16 !458
- chore: Move environment introspection to sshenv module !451
- gitlab-sshd: Acceptance test for the discover command !457
- Replace cleanup functions with t.Cleanup !460 
- Extract sshd connections and sessions into their own files and structs !463