cmd/go: download 1.X.0 instead of 1.X during toolchain upgrade.
This CL modifies the download behavior when downloading a toolchain for 1.21+. Previously, Go would attempt to download 1.X when upgrading the toolchain which would cause the download to fail for 1.21+ since 1.X is an invalid toolchain. We will attempt to download 1.X.0 since that's likely what the user intended.
Additionally, we will also now provide a better error message when the
user provides a language version instead of a toolchain version for
1.21+.
Fixes #66175
Fixes #62278
Change-Id: I28f894290a19d8e3cd220e9d70aeca8f4447e5a1
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/580217 Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>