Help: Update contributor instructions to refer to the forum

Fixes: #20746

CONTRIBUTING.rst

@@ -11,11 +11,11 @@ Community
 
 CMake is maintained and supported by `Kitware`_ and developed in
 collaboration with a productive community of contributors.
-Please subscribe and post to the `CMake Developers List`_ to raise
+Please post to the ``Development`` category of the `CMake Forum`_ to raise
 discussion of development topics.
 
 .. _`Kitware`: http://www.kitware.com/cmake
-.. _`CMake Developers List`: https://cmake.org/mailman/listinfo/cmake-developers
+.. _`CMake Forum`: https://discourse.cmake.org
 
 Patches
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