Given the introduction of currently KIO::WorkerBase & KIO::TCPWorkerBase parallel to the old KIO::SlaveBase & KIO::TCPSlaveBase classes, this task no longer is waiting for KF6 branching.

Instead actually already under process, and has to be completed before KF6 branching, to not end up with a mess of some legacy-named parallel classes still around (e.g. KIO::ForwardingSlaveBase without replacement yet, but subclassing KIO::SlaveBase).

Most of the API using the old term has already be deprecated in favour of "worker"-termed API. Some things though remain, so turning this now into a meta task and adding subtasks for the things that should be done before KF6 branching.

Thanks so much for taking the initiative on this stage of the work, @kossebau! 👍