RFR: build: update to Gradle 7.0 [v2]

[Erik Joelsson]

On Mon, 12 Apr 2021 14:10:34 GMT, Erik Helin <ehelin at openjdk.org> wrote:

>> Hi all,
>> 
>> please review this patch that updates Gradle to version `7.0`. Gradle is a bit stricter in 7.0 when it comes to consuming task output, so I had to ensure that the `Output` properties per task were unique.
>> 
>> Thanks,
>> Erik
>
> Erik Helin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update gradle-wrapper.properties

Marked as reviewed by erikj (Reviewer).

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PR: https://git.openjdk.java.net/skara/pull/1113