cmd/fix: invalid -go=go1.21.0

[alexbozhenko]

According to my understanding of these docs:
https://go.dev/doc/go1.21#tools
https://go.dev/doc/toolchain#version
https://go.dev/ref/mod#go-mod-file-go

before go 1.21 it was not allowed to specify the patch version in the go.mod.
so the code in fix cmd:

go/src/cmd/fix/main.go

Lines 78 to 79 in 905b58b

	if before, after, found := strings.Cut(majorStr, "."); found {
	majorStr, minorStr = before, after

does not handle string 1.21.0.
Existing code also won't work with go 1.21rc1

What version of Go are you using (go version)?

$ go version
go version go1.21.0 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/alex/.cache/go-build'
GOENV='/home/alex/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/alex/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/alex/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.21.0'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/alex/code/reproducer/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2636372290=/tmp/go-build -gno-record-gcc-switches'

What did you do?

$ mkdir reproducer
$ cd reproducer/
$ go mod init reproducer
$ cat go.mod
module reproducer

go 1.21.0

$ cat << EOF > main.go
//go:build OMIT
// +build OMIT

package main

import "fmt"

func main() {
    fmt.Println("Hello, World!")
}
EOF

$ go fix -tags OMIT .
invalid -go=go1.21.0
exit status 2

What did you expect to see?

Expected go fix to remove the deprecated build tag.

What did you see instead?

Error about invalid version.

[heschi]

cc @bcmills @matloob @rsc @ianlancetaylor