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.
Fixes https://bugs.dolphin-emu.org/issues/12245.
I considered making a change to DolphinQt instead of
the core, but then additional effort would've been
required to add the same fix to the Android GUI once
we start using the new config system there.
