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