conn manager: Change header limit field to max_request_headers_kb (#5763)

Renaming max_request_headers_size_kb to max_request_headers_kb.

It just merged so hopefully no one is using it yet.

Risk Level: Low.
Testing/docs: Updated docs and config test from #5654.
Signed-off-by: Auni Ahsan <auni@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 71afb134c528ded99510172a65e30cac6503d213

envoy/config/filter/network/http_connection_manager/v2/http_connection_manager.proto

@@ -136,7 +136,7 @@ message HttpConnectionManager {
   // block in nghttp2 is 64K. The max configurable setting is 63K in order to
   // stay under both codec limits.
   // Requests that exceed this size will receive a 431 response.
-  google.protobuf.UInt32Value max_request_headers_size_kb = 29
+  google.protobuf.UInt32Value max_request_headers_kb = 29
       [(validate.rules).uint32.gt = 0, (validate.rules).uint32.lte = 63];
 
   // The idle timeout for connections managed by the connection manager. The