update Go version to 1.17

[alexandear]

I think it's time to update to Go 1.17. See comments in #1871.

Currently, it's impossible to run tests with Go 1.15:

❯ go1.15 version
go version go1.15 darwin/amd64
❯ go1.15 test ./...
# github.com/spf13/cobra/doc [github.com/spf13/cobra/doc.test]
doc/man_docs_test.go:167:17: undefined: os.MkdirTemp
doc/man_docs_test.go:218:15: undefined: os.CreateTemp
doc/md_docs_test.go:96:17: undefined: os.MkdirTemp
doc/md_docs_test.go:112:15: undefined: os.CreateTemp
doc/rest_docs_test.go:83:17: undefined: os.MkdirTemp
doc/rest_docs_test.go:99:15: undefined: os.CreateTemp
doc/yaml_docs_test.go:60:17: undefined: os.MkdirTemp
doc/yaml_docs_test.go:87:15: undefined: os.CreateTemp
# github.com/spf13/cobra [github.com/spf13/cobra.test]
./cobra_test.go:259:9: undefined: os.WriteFile
./command_test.go:2199:22: undefined: io.ReadAll
./command_test.go:2204:22: undefined: io.ReadAll
./fish_completions_test.go:102:18: undefined: os.CreateTemp
./fish_completions_test.go:121:17: undefined: os.MkdirTemp
FAIL    github.com/spf13/cobra [build failed]
FAIL    github.com/spf13/cobra/doc [build failed]
FAIL
❯ go1.17 test -count=1 ./...
ok      github.com/spf13/cobra  1.041s
ok      github.com/spf13/cobra/doc      0.325s

How to install go1.15

$ GOARCH=amd64 go run golang.org/dl/go1.15@latest download
$ go install golang.org/dl/go1.15@latest

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[ccoVeille]

I'm surprised to see thus one popping here

[alexandear]

It's an indirect dependency of github.com/cpuguy83/go-md2man/v2/md2man:

❯ go1.17 mod why -m github.com/russross/blackfriday/v2
# github.com/russross/blackfriday/v2
github.com/spf13/cobra/doc
github.com/cpuguy83/go-md2man/v2/md2man
github.com/russross/blackfriday/v2