Use `LICENSES/GPL-3.0-or-later` (`GPL-3.0` is deprecated)

Use `LICENSES/GPL-3.0-or-later` (`GPL-3.0` is deprecated)

As of this date, the SPDX website says [1] about "GNU General Public
License v3.0 only":

> This license identifier has been deprecated since license list
> version 3.0.

Therefore, use `GPL-3.0-or-later`.

That's already the short ID we use in `tests/utils/tap/tap.sh`, not
`GPL-3.0`.

[1]: https://spdx.org/licenses/GPL-3.0.html

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I72349b1631cabfa16a59e0228871205978d5dd91
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11378
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>