cmd/go: go mod download needs to support toolchain upgrades [1.21 backport]

[gopherbot]

@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.

[joedian]

@bcmills can you please raise the cl for this?

[bcmills]

I'm working on it today, but if it slips to Go 1.21.2 that's probably fine. (Really we just need it to be released in time for the proxy to upgrade to it while Go 1.20.x is still supported, so that we have time to test the proxy before it migrates.)

[dmitshur]

@bcmills The release branches are planned to be frozen today. Given the upstream issue isn't resolved yet, and you said it might be better to let it slip to Go 1.21.2 instead of holding back the Go 1.21.1 release, I plan to move this to the Go 1.21.2 milestone soon. Please comment if you think another path forward is better.

[bcmills]

Slipping to 1.21.2 sounds good. Thanks for the heads-up.

[thanm]

Hi @bcmills looking at the schedule for Go 1.21.2 I think we are supposed to have all cherry-picks done by this Tues in order to freeze the release branch Wed and pre-announce on Thu. Will the fix for this bug make the release do you think, or should we bump to 1.21.3? Thanks.

[bcmills]

I don't think this will be in by Tuesday, so let's bump to 1.21.3.

[thanm]

OK thanks, I've moved it.