Preserve compatibility with go <= 1.17

[thaJeztah]

• depends on gha: update golangci-lint to v1.62.2 #131
• revert md2man.go: rename ioutil to io #125
• relates to Update to latest go-md2man spf13/cobra#2201 (comment)

Revert "md2man.go: rename ioutil to io" for compat with go <= 1.17

This reverts commit 7607604.

md2man: use local copy of bytes.Copy for compat with go < 1.1

[thaJeztah]

OK; looks like 1.11 doesn't work;

Error: md2man/roff.go:107:28: undefined: bytes.Cut
Error: md2man/roff.go:140:18: undefined: strings.ReplaceAll

• bytes.Cut was added in go1.18; https://pkg.go.dev/bytes#Cut
• strings.ReplaceAll was added in go1.12; https://pkg.go.dev/strings#ReplaceAll

[cpuguy83]

Is this ready, @thaJeztah?

[thaJeztah]

@cpuguy83 if you can approve #131, then I'll rebase this one (CI is currently red due to golangci-lint incompatible with go1.23)