[chain_redesign] Remove old structures

Other changes:

* The `async-https` feature of `bdk_esplora` is no longer default.
* Rename `ObservedAs` to `ChainPosition`.
* Set temporary MSRV to 1.60.0 to compile all workspace members will all
  features.

example-crates/example_electrum/src/main.rs

@@ -13,7 +13,7 @@ use bdk_chain::{
 };
 use bdk_electrum::{
     electrum_client::{self, ElectrumApi},
-    v2::{ElectrumExt, ElectrumUpdate},
+    ElectrumExt, ElectrumUpdate,
 };
 use example_cli::{
     anyhow::{self, Context},