RFR: build: update to Gradle 6.3

[Erik Helin]

Hi all,

please review this patch that updates Gradle to version 6.3.

Testing:
- `sh gradlew images`
- `sh gradlew test`

Thanks,
Erik

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Commit messages:
 - build: update to Gradle 6.3

Changes: https://git.openjdk.java.net/skara/pull/614/files
 Webrev: https://webrevs.openjdk.java.net/skara/614/webrev.00
  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/614.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/614/head:pull/614

PR: https://git.openjdk.java.net/skara/pull/614