subsys/cpp: update tag for C++2a to C++20

C++20 was tagged and released in December of 2020.  It is no longer
forth coming.

Signed-off-by: Dan Kalowsky <dkalowsky@amperecomputing.com>

CMakeLists.txt

@@ -167,8 +167,8 @@ if(CONFIG_CPLUSPLUS)
     set(STD_CPP_DIALECT_FLAGS $<TARGET_PROPERTY:compiler-cpp,dialect_cpp14>)
   elseif(CONFIG_STD_CPP17)
     set(STD_CPP_DIALECT_FLAGS $<TARGET_PROPERTY:compiler-cpp,dialect_cpp17>)
-  elseif(CONFIG_STD_CPP2A)
-    set(STD_CPP_DIALECT_FLAGS $<TARGET_PROPERTY:compiler-cpp,dialect_cpp2a>)
+  elseif(CONFIG_STD_CPP20)
+    set(STD_CPP_DIALECT_FLAGS $<TARGET_PROPERTY:compiler-cpp,dialect_cpp20>)
   else()
     assert(0 "Unreachable code. Expected C++ standard to have been chosen. See Kconfig.zephyr.")
   endif()

subsys/cpp/Kconfig

@@ -37,10 +37,10 @@ config STD_CPP17
 	help
 	  2017 C++ standard, previously known as C++0x.
 
-config STD_CPP2A
-	bool "C++ 2a"
+config STD_CPP20
+	bool "C++ 20"
 	help
-	  Next revision of the C++ standard, which is expected to be published in 2020.
+	  2020 C++ standard, previously known as C++2A.
 
 endchoice