Remove envoy.reloadable_features.upstream_http2_flood_checks runtime override and legacy code paths (#19016)

Remove envoy.reloadable_features.upstream_http2_flood_checks runtime override and legacy code paths.

Risk Level: Low
Testing: Unit Tests
Docs Changes: proto comments updated
Release Notes: Yes
Platform Specific Features: N/A
Fixes #18449

Signed-off-by: Yan Avlasov <yavlasov@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 5d1ed3f9e2e83cd4a773b6f60058c8c1960e6138

envoy/config/core/v3/protocol.proto

@@ -408,8 +408,6 @@ message Http2ProtocolOptions {
   // be written into the socket). Exceeding this limit triggers flood mitigation and connection is
   // terminated. The ``http2.outbound_flood`` stat tracks the number of terminated connections due
   // to flood mitigation. The default limit is 10000.
-  // NOTE: flood and abuse mitigation for upstream connections is presently enabled by the
-  // `envoy.reloadable_features.upstream_http2_flood_checks` flag.
   google.protobuf.UInt32Value max_outbound_frames = 7 [(validate.rules).uint32 = {gte: 1}];
 
   // Limit the number of pending outbound downstream frames of types PING, SETTINGS and RST_STREAM,
@@ -417,8 +415,6 @@ message Http2ProtocolOptions {
   // this limit triggers flood mitigation and connection is terminated. The
   // ``http2.outbound_control_flood`` stat tracks the number of terminated connections due to flood
   // mitigation. The default limit is 1000.
-  // NOTE: flood and abuse mitigation for upstream connections is presently enabled by the
-  // `envoy.reloadable_features.upstream_http2_flood_checks` flag.
   google.protobuf.UInt32Value max_outbound_control_frames = 8 [(validate.rules).uint32 = {gte: 1}];
 
   // Limit the number of consecutive inbound frames of types HEADERS, CONTINUATION and DATA with an
@@ -427,8 +423,6 @@ message Http2ProtocolOptions {
   // stat tracks the number of connections terminated due to flood mitigation.
   // Setting this to 0 will terminate connection upon receiving first frame with an empty payload
   // and no end stream flag. The default limit is 1.
-  // NOTE: flood and abuse mitigation for upstream connections is presently enabled by the
-  // `envoy.reloadable_features.upstream_http2_flood_checks` flag.
   google.protobuf.UInt32Value max_consecutive_inbound_frames_with_empty_payload = 9;
 
   // Limit the number of inbound PRIORITY frames allowed per each opened stream. If the number
@@ -442,8 +436,6 @@ message Http2ProtocolOptions {
   // `opened_streams` is incremented when Envoy send the HEADERS frame for a new stream. The
   // ``http2.inbound_priority_frames_flood`` stat tracks
   // the number of connections terminated due to flood mitigation. The default limit is 100.
-  // NOTE: flood and abuse mitigation for upstream connections is presently enabled by the
-  // `envoy.reloadable_features.upstream_http2_flood_checks` flag.
   google.protobuf.UInt32Value max_inbound_priority_frames_per_stream = 10;
 
   // Limit the number of inbound WINDOW_UPDATE frames allowed per DATA frame sent. If the number
@@ -460,8 +452,6 @@ message Http2ProtocolOptions {
   // flood mitigation. The default max_inbound_window_update_frames_per_data_frame_sent value is 10.
   // Setting this to 1 should be enough to support HTTP/2 implementations with basic flow control,
   // but more complex implementations that try to estimate available bandwidth require at least 2.
-  // NOTE: flood and abuse mitigation for upstream connections is presently enabled by the
-  // `envoy.reloadable_features.upstream_http2_flood_checks` flag.
   google.protobuf.UInt32Value max_inbound_window_update_frames_per_data_frame_sent = 11
       [(validate.rules).uint32 = {gte: 1}];