Merge bitcoindevkit/bdk#1977: Add `TxGraph::get_last_evicted`

Merge bitcoindevkit/bdk#1977: Add `TxGraph::get_last_evicted`

b555acb91e30f7134ec2663477b015225140b11b feat(chain): Add `TxGraph::get_last_evicted` (志宇)

Pull request description:

  ### Description

  Ideally, this would be included as a field in `TxNode`, however that would be a breaking change.

  ### Changelog notice

  ```md
  Added
  - `TxGraph::get_last_evicted`
  ```

  ### 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 +nightly fmt` and `cargo clippy` before committing

  #### New Features:

  ~* [ ] I've added tests for the new feature~
  * [x] I've added docs for the new feature

ACKs for top commit:
  evanlinjin:
    self-ACK b555acb91e30f7134ec2663477b015225140b11b

Tree-SHA512: f1661d204e89e400b93e17f51f890a591edcdeea95803c4c48b937b700f48d5f2d975fcf33def3c5e36df13809ac2cc75f99ffc99716fa38c356b253a61eb5cb