Merge bitcoindevkit/bdk#2450: refactor(chain): use confirmations_lower_bound

ad1cda7e18ac074193365f76e355545ede8cd58a refactor(chain): use confirmations_lower_bound (Dmenec)

Pull request description:

  ### Description

  Uses `ChainPosition::confirmations_lower_bound` in `CanonicalTxOut::is_mature`, `CanonicalTxOut::is_confirmed_and_spendable`, the `CanonicalView::balance` doc example and the `settled` test helper, instead of computing the confirmations by hand from `confirmation_height_upper_bound`.

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] Ran `just p` before committing

ACKs for top commit:
  evanlinjin:
    ACK ad1cda7e18ac074193365f76e355545ede8cd58a

Tree-SHA512: 35212c2df0cb7e61d5b28f8a9d3486aaf40a804b856e9fb1a8a4c9b9969547906171ff4b8165b91eff589e804ba59e234b294b012db34f58de02a711a8beed5c