go.mod: set go version
Change-Id: I45f1816a359ad6fab548338fd3a6b01dda32d01f Reviewed-on: https://go-review.googlesource.com/c/crypto/+/203838 Run-TryBot: Bryan C. Mills <bcmills@google.com> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
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