Incompatible with Gradle 7

[leo60228]

A problem was found with the configuration of task ':compileRetrolambdaMain' (type 'RetrolambdaTask').
  - In plugin 'me.tatarka.RetrolambdaPluginJava' type 'me.tatarka.RetrolambdaTask' property 'classpath' has @Input annotation used on property of type 'FileCollection'.
    
    Reason: A property of type 'FileCollection' annotated with @Input cannot determine how to interpret the file.
    
    Possible solutions:
      1. Annotate with @InputFile for regular files.
      2. Annotate with @InputFiles for collections of files.
      3. If you want to track the path, return File.absolutePath as a String and keep @Input.
    
    Please refer to https://docs.gradle.org/7.6.1/userguide/validation_problems.html#incorrect_use_of_input_annotation for more details about this problem.

This was made a warning in Gradle 6.0 and an error in Gradle 7.0.