Document go1.8.3 as dependency

What does this MR do?

Updated the documentation to state that you're required to have Go 1.8 installed to build certain components of GitLab

Are there points in the code the reviewer needs to double check?

Grammar! 😆 (And if my text makes any sense...)

Why was this MR needed?

Reasoning in https://gitlab.com/gitlab-org/gitlab-ce/issues/31943

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#31943 (closed) gitaly#203 (closed)