[kconf_update] Use scripted dim screen effect

Summary:
I forgot to add a kconf update that moves existing users of the builtin
Dim Screen effect to the scripted version of the effect, sorry.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16957