internal/goversion: update Version to 1.18

This is the start of the Go 1.18 development cycle, so update the
Version value accordingly. It represents the Go 1.x version that
will soon open up for development (and eventually become released).

Updates #40705
Updates #47351

Change-Id: Icfb99e28529a3c9fb7394e114a34586f613dfcbf
Reviewed-on: https://go-review.googlesource.com/c/go/+/341590


Trust: Carlos Amedee <carlos@golang.org>
Run-TryBot: Carlos Amedee <carlos@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Alexander Rakoczy <alex@golang.org>