load balancer: fix?: panic mode is disabled only when healthy_panic_threshold is 0% (#7478)

Currently, in load_balancer_impl.cc: recalculatePerPriorityPanic(), even if common_lb_config.healthy_panic_threshold is 0%, a load balancer enters panic mode whenever normalized_total_availability is 0%.
I guess, a user who intentionally set to healthy_panic_threshold = 0 expects to immediately return error responses if there is no available host checked by a load balancer.
(In fact, current load_balancer_impl.cc: isGlobalPanic() decide not to enter panic mode whenever healthy_panic_threshold is 0%.)
So I suggest that panic mode is disabled only when healthy_panic_threshold is 0%.
I want this change for automatic degenerating lower priority or optional back-end services.

Risk Level: Low
(It seems that setting healthy_panic_threshold == 0 is a special case originally. It won't happen unless a user intend to disable panic mode, because default value of healthy_panic_threshold is 50%.)
Testing: unit and Integration tests with ./ci/run_envoy_docker.sh './ci/do_ci.sh bazel.release'
Docs Changes: inline

Signed-off-by: mnktsts2 <mnktsts2@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ ad9926fb29dc047d7583ab5b0217a3c379f14200

envoy/api/v2/cds.proto

@@ -517,6 +517,7 @@ message Cluster {
   message CommonLbConfig {
     // Configures the :ref:`healthy panic threshold <arch_overview_load_balancing_panic_threshold>`.
     // If not specified, the default is 50%.
+    // To disable panic mode, set to 0%.
     //
     // .. note::
     //   The specified percent will be truncated to the nearest 1%.