scripted-diff: Wallet: Rename incorrectly named *UsedDestination

Summary:

These functions are used to mark/check if a key of our own has been used to spend (and only for avoid-reuse wallets), which has nothing to do with the destination/address itself.
Give them more accurate names to avoid confusion.

-BEGIN VERIFY SCRIPT-
sed -i -e 's/UsedDestination/SpentKey/g' $(git grep -l 'UsedDestination' ./src)
-END VERIFY SCRIPT-

This is a backport of Core PR18193

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8767