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: Ie8a20191fd9505eb5f0a6a3a010a602d2a44a58e

composer.json

@@ -8,7 +8,7 @@
 		"skin"
 	],
 	"homepage": "https://www.mediawiki.org/wiki/Skin:Vector",
-	"license": "GPL-2.0+",
+	"license": "GPL-2.0-or-later",
 	"authors": [
 		{
 			"name": "Trevor Parscal"

skin.json

@@ -8,7 +8,7 @@
 	"url": "https://www.mediawiki.org/wiki/Skin:Vector",
 	"descriptionmsg": "vector-skin-desc",
 	"namemsg": "skinname-vector",
-	"license-name": "GPL-2.0+",
+	"license-name": "GPL-2.0-or-later",
 	"type": "skin",
 	"requires": {
 		"MediaWiki": ">= 1.29.0"