Help: Add release note for C++ 20 support

Fixes: #17849

Help/release/dev/features-c++20.rst

+features-c++20
+--------------
+
+* The :manual:`Compile Features <cmake-compile-features(7)>` functionality
+  is now aware of C++ 20.  No specific features are yet enumerated besides
+  the ``cxx_std_20`` meta-feature.