[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/wallet_esplora/src/main.rs

@@ -10,7 +10,7 @@ use bdk::{
     wallet::AddressIndex,
     SignOptions, Wallet,
 };
-use bdk_esplora::{esplora_client, v2::EsploraExt};
+use bdk_esplora::{esplora_client, EsploraExt};
 use bdk_file_store::Store;
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {