cmd/go: go mod download needs to support toolchain upgrades [1.21 backport] #62055
Description
@bcmills requested issue #62054 to be considered for backport to the next 1.21 minor release.
@gopherbot, please backport to Go 1.21. This is a blocker for the Go module proxy to be able to deploy Go 1.21 releases with adequate pre-release testing.