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

df74b23f315007cc36d3a12904380f861326fb32 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 df74b23f315007cc36d3a12904380f861326fb32

Tree-SHA512: fb11db0771542b074b7605e4af9ec655c22106b1d3451e27103d2f81686270efb44d8687f8312b5b7ab724f03e8f24c349a9f4f3b6e0c89ed9550b121f526ce7