Allow configuration cache compatibility

[jprinet]

• Use FileSystemOperations to delete output folder if Gradle Version >= 6
• Add GenerateTaskConfigurationCacheTest test class

Fixes #15692

Please note that the FileSystemOperations has been introduced with Gradle 6.0 and this is the reason why we need to condition its usage.

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package 
  ./bin/generate-samples.sh
  ./bin/utils/export_docs_generators.sh
  

  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master (6.3.0) (minor release - breaking changes with fallbacks), 7.0.x (breaking changes without fallbacks)

[jprinet]

Just realized that Gradle 5.6.1 used in the newly introduced GenerateTaskConfigurationCacheTest class is not compatible with JDK 17. Let me fix that.

[jprinet]

I found a way but having TestNg to provide a DisabledForJreRange would have greatly helped

[wing328]

cc @OpenAPITools/generator-core-team