cmd/k8s-operator: attach egress readiness gate to ProxyGroup Pods

The egressPodsReconciler (added in #14792) only sets the
tailscale.com/egress-services readiness condition on egress ProxyGroup
replica Pods that declare the corresponding readiness gate. However, the
gate was never actually added to the egress Pod template, so the
reconciler always hit its early-return and the readiness condition was
never set. This commit adds said readiness gate to the egress Pod template.

Updates #14326

Signed-off-by: chaosinthecrd <tom@tmlabs.co.uk>