doc: Replace hidden service with onion service

Summary:
For a couple of years, Tor documentation has made
the term hidden service obsolete, in favor of onion
service.

This PR updates all the references in the code base.

This is a backport of core#19638

Test Plan: proof-reading

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10062