API dox: refer to "worker"s now

Possibly the transition could be done more gracefully and with more explanations, but given the lack of human power, just directly switching the term seems best to me to keep moving.

There are some more non-worker references, but those are in classes which also need adaption of method names to the new worker nomenclature, will do in separate MRs.

@dfaure @nicolasfella @sitter @ahmadsamir