Drop compatibility with pre-2024-08 remoting releases

[daniel-beck]

The fix for SECURITY-3430 retains compatibility with older remoting releases to have a smoother upgrade experience. This PR drops that support. It's probably going to be a while before we want to merge this, but I thought it would be useful to have this prepared.

Testing done

Autotests in jenkinsci/jenkins#9569

Submitter checklist

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[jglick]

(see f9e9393)