Make clear that Go 1.8 is required since GitLab 9.2

What does this MR do?

It seems that Go 1.8 is required to compile gitlab-shell since GitLab 9.2. I've updated and clarified the 9.1->9.2 & 9.2->9.3 upgrade guides.

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

I've removed the sentence about Go 1.5 required to compile gitlab-workhorse since we have a step to install Go above, that was redundant.

What are the relevant issue numbers?

Closes #34525 (closed)