scripted-diff: rename nPeersWithValidatedDownloads

Summary:

-BEGIN VERIFY SCRIPT-
ren() { sed -i "s:\<$1\>:$2:g" $(git grep -l "\<$1\>" ./src ./test); }

ren nPeersWithValidatedDownloads  m_peers_downloading_from
-END VERIFY SCRIPT-

This is a partial backport of core#22141
https://github.com/bitcoin/bitcoin/pull/22141/commits/156a19ee6a22789adedcb6ab067c2eca2d3bfdfe

Depends on D12704

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12706