Maven 4 support requirement

[hazendaz]

[WARNING] Unable to find the root directory. Create a .mvn directory in the root directory or add the root="true" attribute on the root project's model to identify it.

The first item is backwards compatible. The other is not. This is echoed when running a build with maven 4. To fix it, if we have no other reason to start using .mvn that has existed since maven 3.3, then I suggest a simple empty '.gitkeep' to store the folder. That folder location is used by gradle enterprise, develocity, maven.config, jvm.config, and various others so its becoming far more important that using maven wrapper (sic).

Therefore, I will send a PR to add .gitkeep inside .mvn to address the issue as least friction to adhere to this requirement.