Move from using openapi-gen

[kannon92]

Upstream kube has dropped openapi-gen from 1.30.

We should look into using something else. It should no longer be needed.

[kannon92]

We are using the following code:

	$(PROJECT_DIR)/bin/openapi-gen --go-header-file hack/boilerplate.go.txt -i sigs.k8s.io/jobset/api/jobset/v1alpha2 -p sigs.k8s.io/jobset/api/jobset/v1alpha2 --alsologtostderr
	if [ $(GOPATH)/src/sigs.k8s.io/jobset/api/jobset/v1alpha2/openapi_generated.go != $(PROJECT_DIR)/api/jobset/v1alpha2/openapi_generated.go ]; then \
		mv $(GOPATH)/src/sigs.k8s.io/jobset/api/jobset/v1alpha2/openapi_generated.go $(PROJECT_DIR)/api/jobset/v1alpha2/openapi_generated.go; \
	fi

We should be able to swap k8s.io/code-generator/openapi-gen for k8s.io/kube-openapi/openapi-gen.

Going to investigate this.