build: update comments about Go 1.4 in make.bash, make.bat, make.rc

build: update comments about Go 1.4 in make.bash, make.bat, make.rc

A few lingering references to Go 1.4 remain.
Fix them, and document the ones that should stay.

For #44505.
Fixes #54301.

Change-Id: Ieb9c0f08e4d1890ea3fdfd9c8df5493934ae7493
Reviewed-on: https://go-review.googlesource.com/c/go/+/423354
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>