fix flux-github-webhook dependency

[hh]

If we try to apply initially with main.tf / cluster-flux-github-webhook uncommented, we git this error:

╷                                                                                                          
│ Error: Plugin error                                                                                      
│                                                                                                          
│   with module.cluster-flux-github-webhook.kubernetes_manifest.receiver,                                                                                                                     
│   on terraform/flux-github-webhook/main.tf line 16, in resource "kubernetes_manifest" "receiver":        
│   16: resource "kubernetes_manifest" "receiver" {                                                        
│                                                                                                          
│ The plugin returned an unexpected error from plugin.(*GRPCProvider).PlanResourceChange: rpc error: code = Unknown desc = failed to determine resource type ID: failed to look up GVK        
│ [notification.toolkit.fluxcd.io/v1, Kind=Receiver] among available CRDs: customresourcedefinitions.apiextensions.k8s.io is forbidden: User "system:serviceaccount:hh:admin" cannot list reso
│ "customresourcedefinitions" in API group "apiextensions.k8s.io" at the cluster scope                     
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