[backport#16475 2/2] wallet: Rename CWalletKey to OldKey

Summary:
https://github.com/bitcoin/bitcoin/pull/16475/commits/fa6f22bf44c0f741285f27f27ac18e9679802e5e

Depends on D7020

Concludes backport of Core PR16475

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7021