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