Re-license to `GPL-2.0-or-later`

Successor to https://github.com/tuxedocomputers/tuxedo-keyboard/issues/61:

(...) this project's license is not compatible with the Linux kernel's own license, thus the combination is not redistributable.

The Linux kernel is GPLv2 (SPDX: GPL-2.0-only), and the kernel modules are GPLv3+ (SPDX: GPL-3.0-or-later). As GPLv3 is incompatible with GPLv2 when the combination of software cannot be upgraded to GPLv3 on redistribution (as is the case with the Linux kernel), shipping this or getting this merged into the mainline kernel will not be possible without fixing the licensing of these modules first.

Current LICENSE is still GPL-3.0-or-later. Relicensing to GPL-2.0-or-later is a prerequisite to be able to upstream the code into mainline Linux.

In principle, GPL-2.0-or-later should be chosen over GPL-2.0-only as the new license in order to keep the project compatible with software licensed under GPL-3.0-only/GPL-3.0-or-later (even the GPL authors strongly advise to use the *-or-later license versions). In this particular case, it's rather irrelevant, I guess, since the only considerable software to combine tuxedo-drivers with is GPL-2.0-only anyways.