SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.
This literal was deprecated in 5.14.0. Not to mention it wasn't
documented as part of the API either: see the 5.14.0 changelog here:
https://code.qt.io/cgit/qt/qtbase.git/tree/dist/changes-5.14.0?h=v5.14.0
On Qt 6.0 this define is removed entirely. To stay forward compatible,
we can make use of QStringLiteral instead.