deprecate the security class

Summary:
The Security class is not actually used, hasn't seen any active development
in years and uses gpg in a questionable way. Also it is internal and yet
exported.

Fully removable with KF6.

https://mail.kde.org/pipermail/kde-frameworks-devel/2017-January/042011.html

Reviewers: apol

Reviewed By: apol

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D4268