RFR: Update to Gradle 6.4.1

[Erik Helin]

Hi all.

please review this patch that updates Gradle to 6.4.1. I also went over the README to document the versions of tools
used for building and testing.

Testing:
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - Update to Gradle 6.4.1

Changes: https://git.openjdk.java.net/skara/pull/635/files
 Webrev: https://webrevs.openjdk.java.net/skara/635/webrev.00
  Stats: 11 lines in 3 files changed: 1 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/skara/pull/635.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/635/head:pull/635

PR: https://git.openjdk.java.net/skara/pull/635