CreateRequiredWorkflow method should be POST instead of PUT

The [CreateRequiredWorkflow](https://github.com/google/go-github/blob/master/github/actions_required_workflows.go#L97) function specified a `PUT` method. As per the [docs](https://docs.github.com/en/rest/actions/required-workflows?apiVersion=2022-11-28#create-a-required-workflow), the method shoud be `POST`.

I'll happily submit this minor fix unless you folks think otherwise.
