Fix CI: switch to --std=c++20

Author: simonmar

glean.cabal.in

@@ -56,7 +56,7 @@ common fb-haskell
        ghc-options: -O2
 
 common fb-cpp
-  cxx-options: -std=c++17 -Wno-nullability-completeness -fno-omit-frame-pointer
+  cxx-options: -std=c++20 -Wno-nullability-completeness -fno-omit-frame-pointer
   if !flag(clang)
      cxx-options: -fcoroutines
   if flag(asan)