[LibWebRTC] Fix build with CMake >= 3.26

https://bugs.webkit.org/show_bug.cgi?id=254323

Reviewed by NOBODY (OOPS!).

Starting from CMake 3.26, the flags from CMAKE_CXX_STANDARD are moved
before the ones from target_compile_options, so libwebrtc is compiled
in C++11 instead of the default C++20.
It fails in Source/third_party/abseil-cpp/absl/base/policy_checks.h,
which wants C++14.

Source/ThirdParty/libwebrtc/CMakeLists.txt

@@ -2096,7 +2096,6 @@ endif ()
 add_library(webrtc STATIC ${webrtc_SOURCES})
 
 target_compile_options(webrtc PRIVATE
-    "$<$<COMPILE_LANGUAGE:CXX>:-std=gnu++11>"
     "-UHAVE_CONFIG_H"
     "-DWEBRTC_WEBKIT_BUILD=1"
     "-w"