Throughout the project, somehow, we’d copy-and-pasted an invalid SPDX
license expression everywhere. The `+` suffix isn’t understood in SPDX,
it needs to be `or-later` instead.

Fixed using `git search-replace -f "SPDX-License-Identifier:
GPL-2.0\+///SPDX-License-Identifier: GPL-2.0-or-later"`.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>