chore: upgrade gradle

Author: remonh87

example/android/app/build.gradle

@@ -42,7 +42,7 @@ android {
     defaultConfig {
         // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
         applicationId "com.flutter.stripe.example"
-        minSdkVersion 21
+        minSdkVersion flutter.minSdkVersion
         targetSdkVersion 35
         versionCode flutterVersionCode.toInteger()
         versionName flutterVersionName