mlShell should not force ml-gradle 4.3.x into requiring Gradle 6.6 or higher

mlShell is strictly a prototype, but its inclusion in MarkLogicPlugin means that ml-gradle requires Gradle 6.6 or higher due to the ShellTask constructor. 

ShellTask is likely almost never used; the Wiki page for it will be updated so that a user can set mainClass themselves. 

So this ticket will keep mlShell in MarkLogicPlugin, but it will need to have its mainClass configured first. 