Turn on receive_ecn runtime guard (#34031)

Commit Message: Enable the quic_receive_ecn runtime guard.
Additional Description: Enable reporting of incoming Explicit Congestion Notification (ECN) codepoints via QUIC ACK frames.
Risk Level: Low
Testing: Previous PRs implement unit tests for this capability
Docs Changes: N/A
Release Notes: Envoy now reports Explicit Congestion Notification (ECN, RFC 3168, RFC9330) codepoints via QUIC ACK frames.
Platform Specific Features: Windows socket APIs for ECN are different. This is not implemented due to the lack of a Windows CI infrastructure.

Signed-off-by: Martin Duke <martin.h.duke@gmail.com>

source/common/runtime/runtime_features.cc

@@ -76,6 +76,7 @@ RUNTIME_GUARD(envoy_reloadable_features_oauth_use_url_encoding);
 RUNTIME_GUARD(envoy_reloadable_features_original_dst_rely_on_idle_timeout);
 RUNTIME_GUARD(envoy_reloadable_features_proxy_status_mapping_more_core_response_flags);
 RUNTIME_GUARD(envoy_reloadable_features_quic_fix_filter_manager_uaf);
+RUNTIME_GUARD(envoy_reloadable_features_quic_receive_ecn);
 // Ignore the automated "remove this flag" issue: we should keep this for 1 year. Confirm with
 // @danzh2010 or @RyanTheOptimist before removing.
 RUNTIME_GUARD(envoy_reloadable_features_quic_send_server_preferred_address_to_all_clients);
@@ -149,8 +150,6 @@ FALSE_RUNTIME_GUARD(envoy_reloadable_features_quic_defer_logging_to_ack_listener
 FALSE_RUNTIME_GUARD(envoy_reloadable_features_use_config_in_happy_eyeballs);
 // TODO(#33474) removed it once GRO packet dropping is fixed.
 FALSE_RUNTIME_GUARD(envoy_reloadable_features_prefer_quic_client_udp_gro);
-// TODO(martinduke): Delay enabling this guard until all receive_ecn PRs are in place.
-FALSE_RUNTIME_GUARD(envoy_reloadable_features_quic_receive_ecn);
 
 // A flag to set the maximum TLS version for google_grpc client to TLS1.2, when needed for
 // compliance restrictions.