SPDX license expressions

[dg0yt]

I try to understand the licensing to update the summary in the vcpkg port, and I find it somewhat confusing.

The README has

liburing/README

Lines 101 to 104 in 7b32455

	All software contained within this repo is dual licensed LGPL and MIT, see
	COPYING and LICENSE, except for a header coming from the kernel which is
	dual licensed GPL with a Linux-syscall-note exception and MIT, see
	COPYING.GPL and <https://spdx.org/licenses/Linux-syscall-note.html>.

The files have

grep -R -h SPDX buildtrees/liburing/src/buring-2.6-5b246c73a3.clean/ | sort -u
/* SPDX-License-Identifier: (GPL-2.0 WITH Linux-syscall-note) OR MIT */
.\" SPDX-License-Identifier: LGPL-2.0-or-later
/* SPDX-License-Identifier: MIT */
/* SPDX-License-Identifier: MIT */
// SPDX-License-Identifier: MIT

i.e. the claimed "dual licensed LGPL and MIT" isn't reflected in any header.
There is LGPL-2.0-or-later but COPYING is LGPL 2.1.

"GPL-2.0" is a deprecated identifier in SPDX, suffering from the same deficit as the verbal "LGPL" (which even lacks a version, but there is COPYING): It is not clear if means <version>-only or <version>-or-later.
Cf. https://www.gnu.org/licenses/identify-licenses-clearly.html

For reference, Debian has https://tracker.debian.org/media/packages/libu/liburing/copyright-2.6-1.