Use Go generics

#35 bumps the Go version we use in CI to 1.20, so we can migrate the codebase to use some Go generics features.

The obvious change would be to drop `interface{}` in favor of `any`, but it's also worth looking into whether [the pointer logic in `SQL.Query()`](https://github.com/grafana/xk6-sql/blob/39494e48d837aa48c4575c4076bbfcb502b46302/sql.go#L88-L102) could be simplified with some generic type.