Fix compilation error with maven.compiler.release=21

[basil]

While compiling with -Dmaven.compiler.release=21 I got the following compilation failure:

Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.11.0:compile (default-compile) on project jenkins-core: Compilation failure
/home/basil/src/jenkinsci/jenkins/core/src/main/java/hudson/tasks/Publisher.java:[135,24] error: types Comparator<T> and List<E> are incompatible;
  class DescriptorExtensionListImpl inherits unrelated defaults for reversed() from types Comparator and List
  where T,E are type-variables:
    T extends Object declared in interface Comparator
    E extends Object declared in interface List

This is because of the addition of incompatible reversed() methods between these two interfaces. Thinking about this more closely, it makes little sense for anything to implement both List and Comparator so I split out the comparator into its own class to solve the conflict.

Testing done

mvn clean verify -DskipTests -Dmaven.compiler.release=21 -Dmaven.compiler.testRelease=21 failed before this PR and passed after it.

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N/A

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[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!