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: I998ba3c3b3adaf66b7a028d5fab1d7169d595b6c

extension.json

@@ -9,7 +9,7 @@
 	],
 	"url": "https://www.mediawiki.org/wiki/Extension:Math",
 	"descriptionmsg": "math-desc",
-	"license-name": "GPL-2.0+",
+	"license-name": "GPL-2.0-or-later",
 	"callback": "MathHooks::registerExtension",
 	"type": "parserhook",
 	"AutoloadClasses": {