Update Plugin Version

Author: Dani-Koza-AF

package.json

@@ -3,7 +3,7 @@
   "version": "6.15.2",
   "iosSdkVersion": "6.15.3",
   "androidSdkVersion": "6.15.2",
-  "buildNumber": "20",
+  "buildNumber": "21",
   "description": "AppsFlyer SDK plugin for Capacitor",
   "main": "dist/plugin.cjs.js",
   "module": "dist/esm/index.js",