Document supported version(s) of Go

[mattwelke]

A GitHub user notes in #880 (comment) that the presence of 1.21 in https://github.com/kubernetes-sigs/controller-tools/blob/master/go.mod#L3, according to Go conventions, doesn't mean that the project doesn't support any other version of Go than 1.21. It simply means that the Go code contained within it requires at least Go 1.21 to compile.

Of course at the same time, it makes sense that a project may only support up to a certain Go version, because of bugs present in the new version, breaking changes, or simply because the maintainers want to take a more careful approach where they vet new versions of Go first with the code. So stating "only Go 1.21 is supported right now" is perfectly valid.

I propose that information be documented in the README that conveys which Go version(s) are supported (is it just 1.21?).

[sbueringer]

Makes sense to me!

[sbueringer]

Seems like atm we're running tests with Go 1.20? (https://github.com/kubernetes/test-infra/blob/master/config/jobs/kubernetes-sigs/controller-tools/controller-tools-presubmits-master.yaml https://github.com/kubernetes-sigs/controller-tools/blob/master/test.sh)