Review task input declaration

[stefanneuhaus]

Currently the validateTaskProperties task emits a warning:

./gradlew cleanValidateTaskProperties validateTaskProperties

> Task :validateTaskProperties
Task property validation finished with warnings:
  - Warning: Type 'org.rewedigital.frost.tasks.FrostSetupTask': non-property method 'useProxy()' should not be annotated with: @Input

BUILD SUCCESSFUL in 796ms
5 actionable tasks: 2 executed, 3 up-to-date

According to the documentation of the @Input annotation the annotation should be used on properties in Groovy, but we use it on a method in order to achieve lazy configuration.

This task is to review if we still meet the best practices for Lazy Configuration.