conn pool: default enable runtime feature `conn_pool_delete_when_idle` (#17577)
conn pool: default enable runtime feature `conn_pool_delete_when_idle` (#17577)

Signed-off-by: Greg Greenway <f58b4fce410ab27c1d5cc4afd1f2a190e9aa65c3@apple.com>
envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy,lyft/envoy,envoyproxy/envoy