[Rename only] Rename orphan_work_set to m_orphan_work_set.

Summary:
This helps distinguish the member from any local variables.

This is a backport of core#19911 [1/6]
https://github.com/bitcoin/bitcoin/pull/19911/commits/9c47cb29f9f525ee58acc629825a97075156d764

Test Plan: ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10526