Fix version of Gradle in wrapper task

[rweisleder]

Belongs to #306 😉

[ghost]

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[codecholeric]

Thanks for paying attention 😄 I've completely removed the Gradle Wrapper task configuration in 802d90f though, because I think this is a leftover from old times.
When we upgrade the Gradle Wrapper, we'll do it like ./gradlew wrapper --gradle-version=5.6.4 anyway, and afterwards it's checked in, so I don't really see any use to configure this task with a default version at all.