feat(chain)!: rm local_chain::Update

The intention is to remove the `Update::introduce_older_blocks`
parameter and update the local chain directly with `CheckPoint`.

This simplifies the API and there is a way to do this efficiently.

crates/bdk/src/wallet/mod.rs

@@ -107,7 +107,7 @@ pub struct Update {
     /// Update for the wallet's internal [`LocalChain`].
     ///
     /// [`LocalChain`]: local_chain::LocalChain
-    pub chain: Option<local_chain::Update>,
+    pub chain: Option<CheckPoint>,
 }
 
 /// The changes made to a wallet by applying an [`Update`].