Deprecated allowAsDefault

Summary: Execute upon T12309. The KServiceOffer still takes a 'bool allowedAsDefault', that needs an overload, but in a different patch

Test Plan: builds

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: kossebau, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25798