folly-clib: switch to C++20 (#164)

Summary:
Fixes breakage in the Cabal-only CI job

Pull Request resolved: #164

Reviewed By: phlalx

Differential Revision: D81039973

Pulled By: jjuliamolin

fbshipit-source-id: e781f774b621f596b2b9f7ae453bbab9c399cae0

Author: simonmar

folly-clib/folly-clib.cabal.in

@@ -31,7 +31,7 @@ source-repository head
   location: https://github.com/facebook/folly.git
 
 common fb-cpp
-  cxx-options: -std=c++17
+  cxx-options: -std=c++20
   if !flag(clang)
      cxx-options: -fcoroutines
   if arch(x86_64)