[CMake] [ANNOUNCE] CMake mailing list now closed

[Robert Maynard]

As was previously announced, CMake is stopping mailing list usage, and
has transitioned to a Discourse forum (https://discourse.cmake.org).

While new posts to the mailing list are disabled, all previous
discussion will be archived so that the knowledge can be searched
going forward.

Hopefully I will see you all on discourse.cmake.org!