RFR: build: update to Gradle 7.0 [v2]

[Erik Helin]

> 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

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/1113/files
  - new: https://git.openjdk.java.net/skara/pull/1113/files/4fd8578c..75de0947

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1113&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1113&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/1113.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1113/head:pull/1113

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