review gradle wrapper usage

The ASF don't think including gradle-wrapper jars in source releases is a good idea.

https://issues.apache.org/jira/browse/LEGAL-570

* we could remove the gradle wrapper and fix docs to say that the user needs to install gradle themselves if they want to use gradle
* we could try to hack the gradlew scripts to fetch the gradle-wrapper jar but this is harder with Windows (Linux/Mac users can be more readily expected to have wget or curl available)
* we don't release this repo - so we might not be required to get rid of the gradle-wrapper jar

Start by reading the linked LEGAL-570 discussion and reviewing the gradle-wrapper jars, gradlew scripts, and any documentation mentioned by the repository. Determine which of the listed approaches is required for this repository, then confirm that the chosen wrapper or installation guidance satisfies the release policy on supported platforms.

Written by the indexing model from the issue text.