Deprecate "extra" generators in favor of cmake-file-api(7)
The "extra" generators were created in CMake's early years to provide support for users of specific IDEs by directly generating their project files alongside make or ninja files. Nowadays the file-api provides a more generic, maintainable, well-tested, and robust way for IDEs to view CMake project build trees. Deprecate the legacy "extra" generators to encourage the corresponding IDEs to use the file-api.
Fixes: #19090 (closed)