LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section

LICENSES: Add modern form of the LGPL-2.1 tags to the usage guide section

The modern forms of the LGPL-2.1 tags have already been added as valid
a long time ago, see commit bc128349588d5 ("LICENSES/LGPL-2.1: Add
LGPL-2.1-or-later as valid identifiers"). However, the information is
still missing in the "Usage-Guide" section, so it can currently be
confusing whether these tags are allowed in new files or not. Thus
add the tags to the usage guide section, too, to make this clear
(similar to what we do in the GPL-2.0 file already).

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>