RFR: build: update to Gradle 7.0

[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

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Commit messages:
 - build: update to Gradle 7.0

Changes: https://git.openjdk.java.net/skara/pull/1113/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1113&range=00
  Stats: 33 lines in 4 files changed: 5 ins; 18 del; 10 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