update custom-flannel CNI test to use more recent version and remove docker.io

[medyagh]

in the net_test.go

			{"custom-flannel", []string{fmt.Sprintf("--cni=%s", filepath.Join(*testdataDir, "kube-flannel.yaml"))}, "cni", "", "kube-flannel", true},

we install a cutom CNI and it is a very old flanell and sitll using docker.io images, we can use something in GHCR or aws public

      containers:
      - name: kube-flannel
       #image: flannelcni/flannel:v0.20.2 for ppc64le and mips64le (dockerhub limitations may apply)
        image: docker.io/rancher/mirrored-flannelcni-flannel:v0.20.2