adding go 1.20 to tests

[halilylm]

I added go version 1.20 to testing workflow.

[lukebakken]

Thank you!

[slagiewka]

@lukebakken @halilylm if this project is supposed to follow the two latest releases the setup-go action supports stable and oldstable versions to be used instead of regular tags.

[Zerpet]

We try to keep up to date with new versions of Go, and ensure they are compatible. At the same time, we don't want to break existing users unless we really have to. That's one of the reasons we still have some x interface{} instead of x any 🙂 That being said, I like the idea of switching to stable and oldstable tags.