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/esplora/tests/async_ext.rs

@@ -69,7 +69,6 @@ pub async fn test_update_tx_graph_without_keychain() -> anyhow::Result<()> {
         {
             let update_cps = sync_update
                 .local_chain
-                .tip
                 .iter()
                 .map(|cp| cp.block_id())
                 .collect::<BTreeSet<_>>();

crates/esplora/tests/blocking_ext.rs

@@ -67,7 +67,6 @@ pub fn test_update_tx_graph_without_keychain() -> anyhow::Result<()> {
         {
             let update_cps = sync_update
                 .local_chain
-                .tip
                 .iter()
                 .map(|cp| cp.block_id())
                 .collect::<BTreeSet<_>>();