Merge bitcoindevkit/bdk#596: Bump MSRV to 1.56

cca69481eb7edb28532a4ed78185248d410050ac Bump MSRV to 1.56 (Alekos Filini)

Pull request description:

  ### Description

  Following the discussion in #331, bump the MSRV to `1.56`. We already have other PRs bumping it to at least `1.51` (#593), but I'm felling like we are always lagging behind and our CI breaks regularly. As @LLFourn suggested, this PR makes a relatively large bump, hoping this buys us enough time to finish splitting up BDK, which will allow us to have a lower MSRV for the "core" crate.

  ### 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

  #### New Features:

  * [x] I've updated `CHANGELOG.md`

ACKs for top commit:
  notmandatory:
    ACK cca69481eb7edb28532a4ed78185248d410050ac

Tree-SHA512: bc6572dc94e1c4cbb6d21f6e06a9730af5763fb4811311a61a6a6ec850b5a65664a21e4a7070a3ebcd702529fbba97b2e9a43c1277b9b9f092e194f16a39bc1a