Bump the wallet version to indicate support for the pre split keypool.
Also prevents any wallets from upgrading to versions between HD_SPLIT
and PRE_SPLIT_KEYPOOL.
Partial backport of Core PR12560 (4/4)
https://github.com/bitcoin/bitcoin/pull/12560/commits/a8da482a8bc87ff26194612727d4a7b86b2fb60d
Depends on D4206