[#949] downgrade Gradle version to 5.6.4 from 6.1.1
[#949] downgrade Gradle version to 5.6.4 from 6.1.1

Necessary to build on Java 14 and Java 15.

See also https://github.com/gradle/gradle/issues/10248
and https://issues.apache.org/jira/browse/GROOVY-9211
remkop/picocli,remkop/picocli,remkop/picocli,remkop/picocli