Composer: update license to GPL-2.0-or-later

We previously had this as `GPL v2`.

As per Composer documentation [[1]], the referenced license should be
an item in the SPDX Open Source License Registry [[2]].

[1]: https://getcomposer.org/doc/04-schema.md#license
[2]: https://spdx.org/licenses/

composer.json

@@ -20,7 +20,7 @@
             "url": "https://github.com/mantisbt/securimage"
         }
     ],
-    "license": "GPL v2",
+    "license": "GPL-2.0-or-later",
     "authors": [
         {
             "name": "MantisBT Team",