Replace Ant build with gradle

The master branch has been using Gradle for a while. The expectation
at the time was that minor maintenance releases can be done on 2.3.x
with the existing Ant build. However that is no longer possible to
perform a release and we need to perform a 2.3.3 release.

The change mirrors similar changes on the master branch:

https://github.com/spring-projects/spring-webflow/commit/4e81930b723a9ee0387621d6f00415b6bb93b836
https://github.com/spring-projects/spring-webflow/commit/50a1cf1defa8bca03dcb0ea580c1134d042c2fb8

Issue: SWF-1615