RFR: Update to Gradle 6.0

[Erik Helin]

Hi all,

please review this patch that updates Gradle to version 6.0. I had to fix two minor deprecation warnings:
- `runtime` -> `runtimeOnly`
- `version` -> `archiveVersion`

Thanks,
Erik

## Testing
- [x] `sh gradlew test --warning-mode all`
- [x] `sh gradlew images --warning-mode all`
- Various manual testing of different build targets

----------------

Commits:
 - 146cd6f6: Update to Gradle 6.0

Changes: https://git.openjdk.java.net/skara/pull/246/files
 Webrev: https://webrevs.openjdk.java.net/skara/246/webrev.00
  Stats: 4 lines in 3 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/246.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/246/head:pull/246

PR: https://git.openjdk.java.net/skara/pull/246