[net processing] Rename nStartingHeight to m_starting_height

Summary:

Not done as a scripted diff to avoid misnaming the local variable in
ProcessMessage().

Partial backport of core#19829:
https://github.com/bitcoin/bitcoin/pull/19829/commits/78040f91687e7f1986e466d448c9b9530830e9b8

Depends on D10866.

Ref T1696.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D10867