refactor: Rename TorController::target to m_tor_control_center

Summary:
target is a proper name for the onion service target address and port.
This change is required for the following commit.

This is a backport of core#19991 [4/8]
https://github.com/bitcoin/bitcoin/pull/19991/commits/e3f07851f02857b4844fccb2e91070c5cd3aad4d

Depends on D10446

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10447