Merge #990: Pin base64 to 0.21.0 to keep the MSRV to 1.57.0

df74b23 Pin base64 to 0.21.0 to keep the MSRV to 1.57.0 (Daniela Brozzoni)

Pull request description:

  Fixes #986

  ### Changelog notice

  - Pin base64 to 0.21.0

  ### 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:
  notmandatory:
    ACK df74b23

Tree-SHA512: fb11db0771542b074b7605e4af9ec655c22106b1d3451e27103d2f81686270efb44d8687f8312b5b7ab724f03e8f24c349a9f4f3b6e0c89ed9550b121f526ce7

crates/esplora/Cargo.toml

@@ -16,6 +16,8 @@ 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"]