Fix Gradle plugin not working on Gradle 7

[Anuken]

This is a tiny change that fixes a build error that occurs when the RoboVM Gradle plugin is run with Gradle 7.

A sample error:

FAILURE: Build failed with an exception.

* What went wrong:
A problem was found with the configuration of task ':ios:createIPA' (type 'ArchiveTask').
  - Type 'org.robovm.gradle.tasks.ArchiveTask' property 'roboVMLogger' is missing an input or output annotation.
    
    Reason: A property without annotation isn't considered during up-to-date checking.
    
    Possible solutions:
      1. Add an input or output annotation.
      2. Mark it as @Internal.
    
    Please refer to https://docs.gradle.org/7.0.2/userguide/validation_problems.html#missing_annotation for more details about this problem.

As the logger is neither an input nor an output, I chose the second solution.

[tommyettinger]

Seems straightforward. As long as Internal is available with Gradle 6, I don't see an issue. It really is a tiny change, and it seems necessary.

[Anuken]

It looks like @Internal was added in 3.0, so this should be safe to add. I've tested this change in my project and it definitely fixes the error.

[dkimitsa]

thanks, look ok