[UPDATE] Migration to the new Rewarded Ad API

[gumaciel]

OS target (Android/iOS):
Android/iOS

Godot version:
3.1.2/3.2.X

Issue description:
"In iOS SDK 7.41.0, the Mobile Ads SDK started offering a new rewarded ads API. As of 7.57.0, the legacy API is deprecated. This guide is intended to help publishers update to the new rewarded ads API."

The main goal now is to keep the module updated to avoid problems publishing your game on the App Store

Please help us to update: https://developers.google.com/admob/ios/rewarded-migration

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Also need update to Android: https://developers.google.com/admob/android/rewarded-ads