Fix usage of deprecated QSet/QList methods

Summary:
Compiiling with Qt 5.14 produces the following warnings:

.../libdiscover/resources/StandardBackendUpdater.cpp:70:43: warning:

‘QList<T> QSet<T>::toList() const [with T = AbstractResource*]’ is deprecated:
Use values() instead. [-Wdeprecated-declarations]

.../libdiscover/resources/StandardBackendUpdater.cpp:220:48: warning:

‘QSet<T> QList<T>::toSet() const [with T = AbstractResource*]’ is deprecated:
Use QSet<T>(list.begin(), list.end()) instead. [-Wdeprecated-declarations]

Test Plan: Clean build with Qt 5.14 with no warnings. Still builds with Qt 5.12

Reviewers: apol, ngraham

Reviewed By: ngraham

Subscribers: apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D26069