Use `LICENSES/GPL-2.0-only` (`GPL-2.0` is deprecated)

Use `LICENSES/GPL-2.0-only` (`GPL-2.0` is deprecated)

As of this date, the SPDX website says [1] about "GNU General Public
License v2.0 only":

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

Therefore, use `GPL-2.0-only`.

That's already the short ID we use everywhere, not `GPL-2.0`.

[1]: https://spdx.org/licenses/GPL-2.0.html

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ifa073ef2a833f5bc6fb1051d50146ca352965688
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11377
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>