Unauthenticated GitRepo Spec Mutation via Fleet Git Webhook Receiver

A vulnerability was discovered in Fleet's Git webhook receiver (the gitjob webhook service). When a webhook secret is not configured, incoming webhook requests are accepted without verification, and processing a request can change the spec.pollingInterval field of a matching GitRepo resource in any namespace. A caller with network access to the webhook service and no Kubernetes credentials can therefore alter GitRepo configuration outside the namespaces they are authorized for.  This only affects SUSE Rancher Fleet 0.16 before 0.16.2, older versions are not affected.