Merge bitcoindevkit/bdk#1502: chore(chain)!: Rename `Append` to `Merge`

962305f41546613b5b41b4131d4a5594f01a2de0 chore(chain)!: Rename `Append` to `Merge` (Wei Chen)

Pull request description:

  <!-- You can erase any parts of this template not applicable to your Pull Request. -->
  Fixes #1467.

  ### Description

  <!-- Describe the purpose of this PR, what's being adding and/or fixed -->
  Renames the `Append` trait to `Merge`.

  ### Notes to the reviewers

  <!-- In this section you can include notes directed to the reviewers, like explaining why some parts
  of the PR were done in a specific way -->

  ### Changelog notice

  <!-- Notice the release manager should include in the release tag message changelog -->
  <!-- See https://keepachangelog.com/en/1.0.0/ for examples -->
  * Rename `bdk_chain::Append` to `bdk_chain::Merge`.

  ### 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 962305f41546613b5b41b4131d4a5594f01a2de0

Tree-SHA512: 2019d9ed631776850cda39c9a53bdd3660c7f4715d8c418824d8ad13718f2b2fd160159e03fd63743dbf0b9e91cfdfed7e4cd4a591636f67d2aaec419486d136