cmd/go: improve "toolchain not available" message when patch version is missing about go HOT 7 CLOSED

it's worse in the case of no explicit TOOLCHAIN (if you set a toolchain I think it's reasonable that it'd be fully qualified) but completely baffling when happening indirectly through a go.mod entry that breaks all previous use of go x.y before go 1.21

See details in #62278 (comment)

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CC @bcmills and @matloob via https://dev.golang.org/owners

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(CC @samthanawalla)

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There are several approaches to work.

1. Specify a patch version in go directive, like go 1.22.0.
2. Specify a toolchain name in toolchain directive, like toolchain go1.22.0. Note that go1.22 isn't a toolchain name.
3. Run export GOTOOLCHAIN='go1.22.0+auto' for your environments that are trying to install newer go toolchain.

The Go Version, like1.22, is called 'language version'.

Specifying language version in go directive without toolchain directive will make developers which need to download newer toolchain fail. I hope go command or any document notes that.

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Change https://go.dev/cl/580217 mentions this issue: Require patch in GOTOOLCHAIN or download 1.X.0 otherwise.

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Change https://go.dev/cl/583797 mentions this issue: [release-branch.go1.21] cmd/go: download 1.X.0 instead of 1.X during toolchain upgrade.

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Change https://go.dev/cl/583915 mentions this issue: [release-branch.go1.22] cmd/go: download 1.X.0 instead of 1.X during toolchain upgrade.

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