Update default CMAKE_CXX_STANDARD from 17 to 20

Summary: Folly now requires C++20 and uses `consteval` and other C++20 features in its headers. Wangle depends on folly and includes these headers, so it must also compile with C++20. Without this change, the open source getdeps build fails when compiling wangle against folly.

Differential Revision: D96033377

fbshipit-source-id: 48acb5e8c10d3e100082c40cf0cdb6799671ce74

Author: afrind

third-party/wangle/src/wangle/CMakeLists.txt

@@ -33,7 +33,7 @@ set(CPACK_PACKAGE_VERSION_PATCH ${PROJECT_VERSION_PATCH})
 include(CPack)
 
 if(NOT CMAKE_CXX_STANDARD)
-  set(CMAKE_CXX_STANDARD 17)
+  set(CMAKE_CXX_STANDARD 20)
   set(CMAKE_CXX_STANDARD_REQUIRED ON)
   set(CMAKE_CXX_EXTENSIONS OFF)
   message(STATUS "setting C++ standard to C++${CMAKE_CXX_STANDARD}")