Port away from deprecated API on Qt 5.15.0
Summary: Upstream recommends using QLocale for turning dates into strings.
Test Plan: Builds on my system now
Reviewers: Frameworks, davidedmundson, sitter
Reviewed By: davidedmundson, sitter
Subscribers: usta, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D27412