chore: bump minimum go version 1.18

[pcj]

@kyoh86 Your comment on the last PR made me think it would be a good idea to make the minimum go version to 1.18.

[kyoh86]

looppointer does not require go 1.18. But x/tools 0.20 requires it. It's enough.

[pcj]

Note the go directive in the go.mod file does not say anything about a runtime requirement, it's just for compilation.

For example, you can't compile the project with go 1.16.2 anymore:

$ go install golang.org/dl/go1.16.2@latest
$ ~/go/bin/go1.16.2 download
$ ~/go/bin/go1.16.2 install ./cmd/looppointer
go: golang.org/x/tools@v0.2.0 requires
	github.com/yuin/goldmark@v1.4.13: missing go.sum entry; to add it:
	go mod download github.com/yuin/goldmark
$ echo $?
1

Whereas:

$ go install golang.org/dl/go1.18.2@latest
$ ~/go/bin/go1.18.2 download
$ ~/go/bin/go1.18.2 install ./cmd/looppointer
$ echo $?
0

Also, your CI is set to 1.18.

[kyoh86]

📝 The error does not related on this issue.

go: golang.org/x/tools@v0.2.0 requires
github.com/yuin/goldmark@v1.4.13: missing go.sum entry; to add it:
go mod download github.com/yuin/goldmark

go < 1.18, it raises runtime error.

$ ~/go/bin/go1.16.2 install ./cmd/looppointer
$ looppointer .  
looppointer: internal error: package "fmt" without types was imported from "github.com/kyoh86/looppointer"