Merge bitcoindevkit/bdk#577: Deprecate Database::flush() function

Merge bitcoindevkit/bdk#577: Deprecate Database::flush() function

a111d254769bfa698043cb9172a3e1cc84329ee4 Deprecate Database::flush() function (Steve Myers)

Pull request description:

  ### Description

  The Database::flush() function is only needed for the sled database on mobile, instead for mobile use the sqlite database.

  ### Notes to the reviewers

  This PR is in preparation for removing the Database::flush() function. See https://github.com/bitcoindevkit/bdk/pull/575#issuecomment-1082916667.

  After the `release/0.18.0` feature freeze branch is created then #575 should be merged.

  ### 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

  #### New Features:

  * [ ] I've added tests for the new feature
  * [ ] I've added docs for the new feature
  * [x] I've updated `CHANGELOG.md`

ACKs for top commit:
  afilini:
    ACK a111d254769bfa698043cb9172a3e1cc84329ee4

Tree-SHA512: 18434dc95dbef47118a0d4fface908bdf920a7ffcef927b36bb740c15f8efcf11dea9198b364648f16f74aaec4aa18e92a3c5e925299b2f3b9d69e566f89e790