Specify AGPLv3+ (#6546)

* Specify AGPLv3+

Since the documentation doesn't specify you can use Mastodon as AGPLv3 or any later version.

* Use newest version of SPDX AGPLv3+ identifier

package.json

 {
   "name": "mastodon",
-  "license": "AGPL-3.0",
+  "license": "AGPL-3.0-or-later",
   "engines": {
     "node": ">=6"
   },