use CLUSTER_PROVIDED instead of ORIGINAL_DST_LB (istio#16286)

* use CLUSTER_PROVIDED instead of ORIGINAL_DST_LB

When using an original destination cluster, the load balancing policy
flag has no effect, as all requests are forwarded along to their
original destinations. Therefore the load balancing policy used on such
a cluster can _only_ be the original destination policy, making the lb
policy redundant. Envoy has added other cluster types that provide/require
certain LB policies, so they introduced the CLUSTER_PROVIDED policy type
for those situations.

https://www.envoyproxy.io/docs/envoy/latest/intro/deprecated#version-1-12-0-pending

resolves istio#16276

* only use CLUSTER_PROVIDED on versions of istio >= 1.3