Port away from deprecated KBookmarksmenu API

Summary:
Needs to wait until we can depend on 5.65

See D25660 for context

Reviewers: #dolphin, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D25665