Merge bitcoindevkit/bdk#993: Unpin base64

ac3759254a939b01830ac6963b1d5160da01b733 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 ac3759254a939b01830ac6963b1d5160da01b733

Tree-SHA512: de66f4be7c7fd608f5a74e51c5666f4fe6b962c5a8345c08c7b1da2551f81b9adfb28c58f0991ac6c985fa5c31f3eb7d17f99edf136b04945cefa24b28eaee19