cmake: Corrected setting of C++ language standard

* reverted old faulty mechanism to set different
  standard according to the toolchain
* Using -std=gnu++2b now for both gcc and clang
* Added a build test app to check the C++ standard in IDF
* Updated english docs to reflect the change to C++23

docs/en/api-reference/api-conventions.rst

@@ -55,7 +55,21 @@ Most ESP-IDF examples use C99 `designated initializers`_ for structure initializ
         /* Correct, fields .arg and .name are zero-initialized */
     };
 
-C++ language doesn't support the designated initializers syntax until C++20. However, the GCC compiler partially supports it as an extension. When using ESP-IDF APIs in C++ code, you may consider using the following pattern::
+The C++ language supports designated initializers syntax, too, but the initializers must be in the order of declaration. When using ESP-IDF APIs in C++ code, you may consider using the following pattern::
+
+    /* Correct, fields .dispatch_method, .name and .skip_unhandled_events are zero-initialized */
+    const esp_timer_create_args_t my_timer_args = {
+        .callback = &my_timer_callback,
+        .arg = &my_arg,
+    };
+
+    ///* Incorrect, .arg is declared after .callback in esp_timer_create_args_t */
+    //const esp_timer_create_args_t my_timer_args = {
+    //    .arg = &my_arg,
+    //    .callback = &my_timer_callback,
+    //};
+
+For more information on designated initializers, see :ref:`Designated initializers <cplusplus_designated_initializers>`. Note that C++ language versions older than C++20 (not the default in the current version of ESP-IDF) do not support designated initializers. If you have to compile code with an older C++ standard than C++20, you may use GCC extensions to produce the following pattern::
 
     esp_timer_create_args_t my_timer_args = {};
     /* All the fields are zero-initialized */