[GStreamer][WebRTC] Fix build with GStreamer 1.18

In downstream WPE, we build with GstWebRTC support by using GStreamer
1.18 + backports for GstWebRTC from WebPlatformForEmbedded/buildroot#550

Author: cadubentzen

Source/WebCore/Modules/mediastream/gstreamer/GStreamerDataChannelHandler.cpp

@@ -262,13 +262,6 @@ bool GStreamerDataChannelHandler::checkState()
 
     RTCDataChannelState state;
     switch (channelState) {
-#if !GST_CHECK_VERSION(1, 22, 0)
-    // Removed in https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2099. In
-    // GStreamer < 1.22 GST_WEBRTC_DATA_CHANNEL_STATE_NEW had the 0 value. We keep this case only to
-    // avoid adding a default case.
-    case GST_WEBRTC_DATA_CHANNEL_STATE_NEW:
-        break;
-#endif
     case GST_WEBRTC_DATA_CHANNEL_STATE_CONNECTING:
         state = RTCDataChannelState::Connecting;
         break;

Source/cmake/GStreamerChecks.cmake

@@ -51,8 +51,9 @@ if (ENABLE_VIDEO OR ENABLE_WEB_AUDIO)
               message(FATAL_ERROR "GStreamerTranscoder >= 1.20 is needed for USE_GSTREAMER_TRANSCODER.")
           endif ()
 
-          if (USE_GSTREAMER_WEBRTC AND (PC_GSTREAMER_VERSION VERSION_LESS "1.20" OR NOT PC_GSTREAMER_WEBRTC_FOUND))
-              message(FATAL_ERROR "GStreamerWebRTC >= 1.20 is needed for USE_GSTREAMER_WEBRTC.")
+          #  We use GStreamer 1.18 with backports for GstWebRTC support. In upstream WebKit, >= 1.20 is required.
+          if (USE_GSTREAMER_WEBRTC AND (PC_GSTREAMER_VERSION VERSION_LESS "1.18" OR NOT PC_GSTREAMER_WEBRTC_FOUND))
+              message(FATAL_ERROR "GStreamerWebRTC >= 1.18 is needed for USE_GSTREAMER_WEBRTC.")
           endif ()
       endif ()
 endif ()