Merge bitcoindevkit/bdk#2011: fix(electrum): fix stale anchor hash on reorg

fdec863ed134f2b94186a3c5d462a5d1e65956ab fix(electrum): fix stale anchor hash on reorg (Wei Chen)

Pull request description:

  ### Description

  Fixes an issue in `batch_fetch_anchors()` in `bdk_electrum` where, if a stale block header was detected and a fresh header was fetched, the confirmation anchor inserted into `anchor_cache` still used the hash from the original stale header instead of the new one.

  ### Changelog notice

  - `batch_fetch_anchors()` no longer uses a potentially stale hash as the anchor.

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] I've added tests to reproduce the issue which are now passing
  * [ ] I'm linking the issue being fixed by this PR

ACKs for top commit:
  evanlinjin:
    ACK fdec863ed134f2b94186a3c5d462a5d1e65956ab

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