Drop support for Go 1.9 in GitLab 11.4

What does this MR do?

Go 1.9 is no longer supported by the Go project. Most of our documentation already tells people who install GitLab to use Go 1.10. This MR fixes the one place where we were still signaling that Go 1.9 is OK: the 11.3-to-11.4 update guide.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51982

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides