RFR: build: update to Gradle 6.6

Tue Aug 18 13:03:19 UTC 2020

On Tue, 18 Aug 2020 12:53:21 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this patch that updates Gradle to version 6.6.
> 
> Testing:
> - [x] `make test` passes on Linux x64
> - [x] `make images` passes on Linux x64
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/722