runtime: envoy.reloadable_features.use_observable_cluster_name removal (#19475)

See #15139 ([cluster] Use alt_stat_name for general observability purposes (access log, tracing, admin)),
which introduced a runtime guarded feature, which has been enabled by default for  6 months, so remove
the old code path.

Risk Level: Low
Testing: n/a
Docs Changes: updated
Release Notes: Deprecate envoy.reloadable_features.use_observable_cluster_name.
Platform Specific Features: n/a
Signed-off-by: Loong <loong.dai@intel.com>

Mirrored from https://github.com/envoyproxy/envoy @ 198ae65234223f48cbe2284eb145e16598593963

envoy/config/cluster/v3/cluster.proto

@@ -735,11 +735,9 @@ message Cluster {
   // emitting stats for the cluster and access logging the cluster name. This will appear as
   // additional information in configuration dumps of a cluster's current status as
   // :ref:`observability_name <envoy_v3_api_field_admin.v3.ClusterStatus.observability_name>`
-  // and as an additional tag "upstream_cluster.name" while tracing. Note: access logging using
-  // this field is presently enabled with runtime feature
-  // `envoy.reloadable_features.use_observable_cluster_name`. Any ``:`` in the name will be
-  // converted to ``_`` when emitting statistics. This should not be confused with :ref:`Router
-  // Filter Header <config_http_filters_router_x-envoy-upstream-alt-stat-name>`.
+  // and as an additional tag "upstream_cluster.name" while tracing. Note: Any ``:`` in the name
+  // will be converted to ``_`` when emitting statistics. This should not be confused with
+  // :ref:`Router Filter Header <config_http_filters_router_x-envoy-upstream-alt-stat-name>`.
   string alt_stat_name = 28 [(udpa.annotations.field_migrate).rename = "observability_name"];
 
   oneof cluster_discovery_type {