Bump go version to at least 1.18?

[rszyma]

I'd like to propose a PR that utilizes generics, but generics are only supported since 1.18, while this lib is 1.16.

Is there any reason to keep at 1.16?

[pelletier]

My only hard rule is to support at least the last two Go versions. In practice, I avoid breaking compatibility without a good reason. I am open to bumping the minimum required Go version to 1.18 to start using generics, but I'd like to see examples of how they improve the library.