go: Correct file name for zsh completions

Previously, the completion file was installed as "_go", which by
convention means that it will be loaded by compinit.  But this file
does not have the #compdef tag, so this doesn't work, and nothing in
the file indicates that it is supposed to be used that way.

Therefore, install the file under the original name "go" to indicate
that you need to load it manually (as the comment in the file
suggests).

Closes #18752.