Update CXX standard to 20

We will abandon ubuntu 20.04 and move to an new ubuntu. CI currently is on 22.04, though not sure if we want to jump to 24.04.

Some future plans on C++20 usage:
1. std::filesystem::path overloads in StandardPath
2. std::format in place of fmt
3. char8_t where appropriate
4. coroutine in some place for async data loading 
5. std::source_location
6. Heterogeneous lookup

Author: wengxt

cmake/Fcitx5CompilerSettings.cmake

@@ -1,7 +1,7 @@
 
 set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_C_STANDARD_REQUIRED TRUE)
 set(CMAKE_C_STANDARD 99)