Fix usage of Derek features

- The features in .DEREK.yml were not correctly stated as per
the user guide. They caused no harm, but may have been misleading
the features removed are all part of the "comments" feature.
- The new "pr_description_required" is turned on which helps
reduce maintainer fatigue when a contributor deletes the whole
text area / description of a new PR. Derek will advise as per:
https://github.com/alexellis/derek/blob/master/USER_GUIDE.md
- This version of Derek also includes the ability to use a slash
command in place of the text "Derek" i.e. "/add label" or
"/set milestone"

Signed-off-by: Alex Ellis <[email protected]>

.DEREK.yml

@@ -15,5 +15,4 @@ maintainers:
 features:
  - dco_check
  - comments
- - labels
- - milestone
+ - pr_description_required