Release v9.0.0

In CHANGELOG:

  - Add a Go implementation of the "discover" command !283
  - Allow Go implementation to get the .gitlab_shell_secret from config !284
  - Add a Go implementation of the 2fa_recovery_codes" command !285
  - Display console messages, if available !287
  - Allow the post-receive hook to print warnings !288
  - Remove hooks, they belong to Gitaly now !291

Not in CHANGELOG:

  - Better testing for JSON deserialisation !282
  - Add pry gem to allow easy debugging !289
  - Explicitly include gitlab_access_status.rb !292