Upgraded GWT fork to 2.12.2

[axeluhl]

Our GWT fork from https://github.com/SAP/gwt-forward-serialization-rpc has been ported to the latest official GWT release 2.12.2. See https://github.com/SAP/gwt-forward-serialization-rpc/tree/issue-7987-gwt-2.12.2.

This repo has worked this in on branch bug6177 which has just completed successful builds, releases and deploy/test runs and hence was merged into main.

Please make sure you upgrade your dev environment to the respective GWT version which you find at https://github.com/SAP/gwt-forward-serialization-rpc/releases/download/gwt-2.12.2/gwt-2.12.2.zip and https://static.sapsailing.com/gwt-2.12.2 zip.