Convert to Go modules and retract v7.1.2

[mellis]

I converted the Go portion of this repository to modules and retracted the previously incorrectly published version v7.1.2+incompatible

I did this by running the following

go mod init github.com/twitchtv/twirp-ruby
go mod tidy
go mod edit -retract "v7.1.2+incompatible"
go mod vendor
go mod tidy

Then verified with

go test ./...
go install ./...

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[marioizquierdo]

LGTM! Thank you so much 🙏