Merge bitcoindevkit/bdk-cli#59: Update MSRV to current stable, 1.56.1

4fd219e47004188ea7f504422790d634896bd3bd Pin version of fd-lock to 3.0.0 (Steve Myers)
6435704815a702345647395bc37704bf26b3bd97 Remove rust-toolchain file (Steve Myers)
8eec674d768ec8d23c37b0fbdee6d648083d2b42 Update MSRV to current stable, 1.56.1 (Steve Myers)

Pull request description:

  ### Description

  Update MSRV to the current stable, 1.56.1 and update the `cargo.lock` file.  Also removed the `rust-toolchain` file.

  ### Notes to the reviewers

  I removed the `rust-toolchain` file because I don't think we need to lock users into a particular version (or force them to override it), and this file seem to primarily be used for projects using a specific nightly release, see: https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file.

  I also had to pin the `fd-lock` dependency because of a recent update to that crate to use the [`rustdoc::missing_doc_code_examples` feature which is only supported by `nightly`](https://doc.rust-lang.org/rustdoc/lints.html#missing_doc_code_examples) :disappointed: .

  ### Checklists

  #### All Submissions:

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

Top commit has no ACKs.

Tree-SHA512: b43d314f014a63cb09b77adc3434e8d5ac3328bfa16ce700195444c72bab1bfc5fecb0a2088cbe2ca6ee86af9283559e51f743f69791768679546871c718fcd8