Merge #993: Unpin base64

ac37592 Unpin base64 (Daniela Brozzoni)

Pull request description:

  base64 lowered the MSRV to 1.57.0 in version 0.21.2

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  evanlinjin:
    ACK ac37592

Tree-SHA512: de66f4be7c7fd608f5a74e51c5666f4fe6b962c5a8345c08c7b1da2551f81b9adfb28c58f0991ac6c985fa5c31f3eb7d17f99edf136b04945cefa24b28eaee19

Author: danielabrozzoni

crates/esplora/Cargo.toml

@@ -16,8 +16,6 @@ bdk_chain = { path = "../chain", version = "0.4.0", features = ["serde", "minisc
 esplora-client = { version = "0.3", default-features = false }
 async-trait = { version = "0.1.66", optional = true }
 futures = { version = "0.3.26", optional = true }
-# base64 versions after 0.21.0 have MSRV 1.60.0
-base64 = "=0.21.0"
 
 [features]
 default = ["async-https", "blocking"]