Renaming clarifies that m_extkey is actually the root
extkey that keys are derived from.
Backport of Core [[https://github.com/bitcoin/bitcoin/pull/18204 | PR18204]] part [3/7] : https://github.com/bitcoin/bitcoin/pull/18204/commits/66c2cadc91d26074b89e5ada68350b5c8676efac
Depends on D8382