kconfig update script for enabled runners

Summary:
migrate the enabled runners from enabledCategories
to the more classic [Plugins] group
the krunner framework supports both, so as long
the enabledCategories entry isn't there,
it will work correctly with the other config format

Test Plan: ran on an existing krunnerrc with different sets of categories enabled, tested krunner and its kcm with it, all expected runner plugins were used both in the kcm and while searching with krunner

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D2873