Port from deprecated KIconLoader::IconSize()

Summary: `IconSize()` method is deprecated from Frameworks 5.66

Reviewers: #dolphin, elvisangelaccio, meven

Reviewed By: #dolphin, elvisangelaccio

Subscribers: kfm-devel

Tags: #dolphin

Maniphest Tasks: T11637

Differential Revision: https://phabricator.kde.org/D26237