Replace latest block regex with latest_checkpoint

[ItoroD]

Currently, in WalletViewModel we use a regex (Regex("""\d+/\d+""")) to get the latest block mined from the logger output. Its best to get this from latest_checkpoint in wallet. This update can be done when lastest_checkpoint is merged to bdk-ffi and updated version is available here (devkit-wallet).

This issue is just to keep track of this so we remember to make the update later.