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: I0b4d6b5eb3e210a131937e65268091ec83f0b46c

composer.json

@@ -8,7 +8,7 @@
 		"ImageMap"
 	],
 	"homepage": "https://www.mediawiki.org/wiki/Extension:ImageMap",
-	"license": "GPL-2.0+",
+	"license": "GPL-2.0-or-later",
 	"require": {
 		"php": ">=5.4",
 		"composer/installers": "1.*,>=1.0.1"

extension.json

@@ -3,7 +3,7 @@
 	"author": "Tim Starling",
 	"url": "https://www.mediawiki.org/wiki/Extension:ImageMap",
 	"descriptionmsg": "imagemap_desc",
-	"license-name": "GPL-2.0+",
+	"license-name": "GPL-2.0-or-later",
 	"type": "parserhook",
 	"MessagesDirs": {
 		"ImageMap": [