Help: Replace 'Dart' with 'CDash' in ctest.1 manual

The modern dashboard server is CDash, not Dart, so update the manual
to talk about CTest as a CDash client.  Also drop documentation for
a Dart2-specific option.
Merge topic 'doc-drop-dart' into next

b8aa0cdf Help: Replace 'Dart' with 'CDash' in ctest.1 manual