build: Requires C++-20

`std::is_standard_layout_v` (used in `util/no_destructor.h`) is not
available until C++-17. This breaks building with some compilers.

Tested with clang 20.1.6.

google#1246
https://en.cppreference.com/w/cpp/types/is_standard_layout.html

Author: davidgumberg

CMakeLists.txt

@@ -11,8 +11,8 @@ set(CMAKE_C_STANDARD 11)
 set(CMAKE_C_STANDARD_REQUIRED OFF)
 set(CMAKE_C_EXTENSIONS OFF)
 
-# This project requires C++11.
-set(CMAKE_CXX_STANDARD 11)
+# This project requires C++17.
+set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)