Cleanup prow/boskos and related folders

[upodroid]

What should be cleaned up or changed:
Delete the following folders:

• prow folder
• boskos folder
• ghproxy
• pkg/genyaml folder
• pkg/flagutil

Some repos haven't updated their dependencies for a while, so open an issue linking to the prow changes and to this issue.

All references to k8s.io/test-infra/prow, k8s.io/test-infra/pkg/genyaml, anywhere in kubernetes/kubernetes-sigs repository.

Provide any links for context:

• https://cs.k8s.io/?q=k8s.io%2Ftest-infra%2Fprow&i=nope&literal=nope&files=&excludeFiles=&repos=