Upgrade Gradle to `6.9.1`
Upgrade Gradle to `6.9.1`

Last of the `v6` series; to help review deprecations for v7 further

GaneshSPatil/gocd,ketan/gocd,GaneshSPatil/gocd,ketan/gocd,Skarlso/gocd,GaneshSPatil/gocd,ketan/gocd,ketan/gocd,GaneshSPatil/gocd,gocd/gocd,gocd/gocd,GaneshSPatil/gocd,ketan/gocd,gocd/gocd,gocd/gocd,Skarlso/gocd,GaneshSPatil/gocd,ketan/gocd,Skarlso/gocd,Skarlso/gocd,Skarlso/gocd,gocd/gocd,gocd/gocd,Skarlso/gocd