Use SPDX 3.0 license identifier

SPDX released version 3 of their license list (<https://spdx.org/licenses/>),
which changed the FSF licenses to explicitly end in -only or -or-later
instead of relying on an easy to miss + symbol.

Bug: T183858
Change-Id: I1d56eb47773159c51782ff32ae4f52d2c99c8225

skin.json

@@ -5,7 +5,7 @@
 	"url": "https://www.mediawiki.org/wiki/Skin:Timeless",
 	"descriptionmsg": "timeless-desc",
 	"namemsg": "skinname-timeless",
-	"license-name": "GPL-2.0+",
+	"license-name": "GPL-2.0-or-later",
 	"type": "skin",
 	"ValidSkinNames": {
 		"timeless": "Timeless"