Use methods in KF5-activities to switch to previous/next activity

Summary:
Since the release of KDE Frameworks 5.68.0 we can now
use the methods exposed in KActivities::Controller to trigger a
switch to the previous or next activity from ShellCorona.

Reviewers: ngraham, ivan, apol

Reviewed By: ngraham, apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28083