[sso-admin] Add license entry to package.json

This was the intended license as stated in all source files.

This commit silences a warning when building the corresponding docker
images.

dd-sso/admin/src/admin/package.json

@@ -2,5 +2,6 @@
   "dependencies": {
     "gentelella": "^1.4.0",
     "socket.io": "^4.1.3"
-  }
+  },
+  "license": "AGPL-3.0-or-later"
 }