go/types: unexport the GoVersion configuration option for Go 1.17

The GoVersion field was added to types.Config as part of the work on
type parameters. Specifically, it was added to be consistent with
cmd/compile/internal/types2, which requires such an option.

This configuration option is useful, but is also non-trivial and did not
go through the proposal process. Unexport it for Go 1.17; we can create
a proposal to export it for Go 1.18.

Fixes #46296

Change-Id: Id82d8a7096887dcfc404c4d6d8da9c761b316609
Reviewed-on: https://go-review.googlesource.com/c/go/+/323430


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>