Upgrade to Gradle 6.6

[mark-vieira]

Gradle 6.6 is GA so we should go ahead and upgrade. Of note, it adds first-class support for the javac --release flag so we can finally address this long standing todo.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Build)

[mark-vieira]

@breskeby I think you've already done some work here, mind going ahead and bumping the wrapper and addressing the TODO linked above?

[breskeby]

will do. the update was done already here: #59909

[breskeby]

@mark-vieira I've updated the build to use the native --release support here: #61093

[mark-vieira]

will do. the update was done already here: #59909

Doh!

[breskeby]

The usage of the --release flag support was added here: #61093 closing this issue