Update Go version in prepare.md

What does this Merge Request do?

When installing the GDK, I got the following notice:

$ rbenv exec gdk install                                                                                                                                                   
(in /Users/pedropombeiro/src/gitlab.org/gitlab-development-kit)
Go version 1.13.8 detected, please install Go version 1.14 or higher.
make: *** [preflight-checks] Error 1

So I assume that the documentation is out of date. This MR updates the documentation to recommend Go 1.14.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.