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