KIC 2.0: handle stderrthreshold flag

[rainest]

Follow up from #1294 (review)

Although the controller's logger doesn't make use of the --stderrthreshold flag, its client-go instance uses klog and does. However, Cobra doesn't play nicely with it; we need to figure out how to make it handle the klog flags properly.

[rainest]

Hmm, so maybe not as easy as I thought. 852a8f5 is based off a combination of
https://flowerinthenight.com/blog/2019/02/05/golang-cobra-klog
https://pkg.go.dev/github.com/spf13/pflag#readme-supporting-go-flags-when-using-pflag
https://pkg.go.dev/k8s.io/klog#readme-how-to-use-klog

But this results in a conflict:

$ ./bin/manager --help
 flag redefined: kubeconfig
panic:  flag redefined: kubeconfig

Removing our kubeconfig definition still overwrites Cobra's generated help with the klog flag set's help, and I'm not sure how we pull out the kubeconfig flag for use with controller-runtime init :(

[shaneutt]

After reading through this and given the complexities and cost-benefit considering that we've moved away from the previous logging mechanisms that employed this, I have opted to deprecate the flag but make it available for compatibility reasons in 2.0 #1512