Use `LICENSES/LGPL-2.1-only` (`LGPL-2.1` is deprecated)

Use `LICENSES/LGPL-2.1-only` (`LGPL-2.1` is deprecated)

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

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

Therefore, use `LGPL-2.1-only`.

That's already the short ID we use in `src/common/list.h`.

[1]: https://spdx.org/licenses/LGPL-2.1.html

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I2a6193f9b9b03275505424ecebc9896d9faabb0e
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11376
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Michael Jeanson <mjeanson@efficios.com>