Removes use of designated initializers in nghttp2_callbacks.cc.

Designated initializers are a C++20 feature, which is not compatible with Envoy builds.

PiperOrigin-RevId: 400215984
diff --git a/http2/adapter/nghttp2_callbacks.cc b/http2/adapter/nghttp2_callbacks.cc
index 4345f5f..648a521 100644
--- a/http2/adapter/nghttp2_callbacks.cc
+++ b/http2/adapter/nghttp2_callbacks.cc
@@ -101,8 +101,7 @@
           nghttp2_settings_entry entry = frame->settings.iv[i];
           // The nghttp2_settings_entry uses int32_t for the ID; we must cast.
           visitor->OnSetting(Http2Setting{
-              .id = static_cast<Http2SettingsId>(entry.settings_id),
-              .value = entry.value});
+              static_cast<Http2SettingsId>(entry.settings_id), entry.value});
         }
         visitor->OnSettingsEnd();
       }