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: I957eec9c5df224e367233c9ce23b9631f04c7458

skin.json

@@ -503,7 +503,7 @@
 		}
 	},
 	"descriptionmsg": "minerva-skin-desc",
-	"license-name": "GPL-2.0+",
+	"license-name": "GPL-2.0-or-later",
 	"manifest_version": 1,
 	"name": "MinervaNeue",
 	"namemsg": "skinname-minerva",