Update controller-gen from 0.15.0 to 0.18.0

After updating the k8s.io related mods when we tried to generate the
manifests using make manifests it failed with

```
/Users/vivek.s/go/pkg/mod/k8s.io/[email protected]/core/v1/doc.go:21:1: missing argument "" (at <input>) /Users/vivek.s/go/pkg/mod/k8s.io/[email protected]/core/v1/doc.go:21:1: missing argument "" (at <input>) k8s.io/api/core/v1:-: unknown type "k8s.io/api/core/v1".LocalObjectReference
```

and other similar issues. It simply meant that the controller-gen version is not compatible with the k8s.io/api version that we  have upgraded it to.
That's why this commit updates the controller-gen version to 0.18.0.

Author: viveksinghggits

Makefile

@@ -359,7 +359,7 @@ docker-push:
 # Download controller-gen locally if necessary
 CONTROLLER_GEN = $(shell pwd)/bin/controller-gen
 controller-gen:
-	$(call go-install-tool,$(CONTROLLER_GEN),sigs.k8s.io/controller-tools/cmd/controller-gen@v0.15.0)
+	$(call go-install-tool,$(CONTROLLER_GEN),sigs.k8s.io/controller-tools/cmd/controller-gen@v0.18.0)
 
 # Download kustomize locally if necessary
 KUSTOMIZE = $(shell pwd)/bin/kustomize