LICENSES: Clarify that the SPDX tag GPL-2.0 is deprecated

From https://spdx.org/licenses/GPL-2.0.html, the SPDX tag GPL-2.0
is deprecated. Instead, GPL-2.0-only should be used.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>

LICENSES/GPL-2.0

@@ -8,9 +8,9 @@ Usage-Guide:
   tag/value pairs into a comment according to the placement
   guidelines in the licensing rules documentation.
   For 'GNU General Public License (GPL) version 2 only' use:
-    SPDX-License-Identifier: GPL-2.0
-  or
     SPDX-License-Identifier: GPL-2.0-only
+  or (now deprecated)
+    SPDX-License-Identifier: GPL-2.0
   For 'GNU General Public License (GPL) version 2 or any later version' use:
     SPDX-License-Identifier: GPL-2.0+
   or