Golang cmd/go: "get toolchain@latest" should ignore release candidates [1.24 backport]

@matloob requested issue #74786 to be considered for backport to the next 1.24 minor release.

@gopherbot please backport 1.24 1.25

I think we should backport this into 1.24. It's too late to get it into 1.25.0 but we should backport it into 1.25.1.

Comment From: mrkfrmn

Taking a look.

Comment From: gopherbot

Change https://go.dev/cl/691957 mentions this issue: [release-branch.go1.24] cmd/go/internal/gover: fix ModIsPrerelease for toolchain versions