sshproxy v2.1.1

What's Changed

• Upgrade vendors by @cyrilst in #61
• github-actions: upgrade checkout and setup-go by @cyrilst in #62
• github-actions: upgrade upload-artifact and download-artifact by @cyrilst in #64
• bugfix: don't crash when a user has no group. Instead display an error by @cyrilst in #63 (bug reported by @od-cea)