Release 2.4.0

## Create a new minor release

### Summary

This release focuses on back porting migration support and event system enhancements from the 3.0 release. Key additions include `get_pre_v1_wallet_keychains` migration helper for upgrading from pre-1.0 bdk sqlite databases, new event tracking methods (`apply_unconfirmed_txs_events`, `apply_evicted_txs_events`), and an `events_helper` function for advanced event handling. The `wallet::signer` module deprecation has been reverted until the new PSBT signer is ready.

### Commit

<--latest commit ID to include in this release-->

### Changelog

#### Added

- Add `get_pre_v1_wallet_keychains` migration helper to assist with migrating from pre-1.0 bdk sqlite databases #365, #415
- Add `Wallet::apply_unconfirmed_txs_events` method for tracking unconfirmed transaction events #399
- Add `Wallet::apply_evicted_txs_events` method for tracking evicted transaction events #399
- Add `Wallet::events_helper` method for advanced users to generate `WalletEvent`s after custom operations #399

#### Reverted

- Un-deprecate `wallet::signer` module #405

### Checklist

Release numbering must follow [Semantic Versioning]. These steps assume the current `release/2.x`
branch version is *2.3.0*.

#### On the day of the release

Tag and publish new release:

- [x] Bump the `release/2.x` branch to `2.4.0` version.
- Change the `Cargo.toml` version value to `2.4.0`.
- The commit message should be "Bump version to 2.4.0".
- [x] Add a tag to the `HEAD` commit in the `release/2.x` branch.
- The tag name should be `v2.4.0`
- The first line of the tag message should be "Release 2.4.0".
- In the body of the tag message put a copy of the **Summary** and **Changelog** for the release.
- Make sure the tag is signed, for extra safety use the explicit `--sign` flag.
- [x] Wait for the CI to finish one last time.
- [x] Push the new tag to the `bitcoindevkit/bdk` repo.
- [x] Publish **all** the updated crates to crates.io.
- [x] Create the release on GitHub.
- Go to "tags", click on the dots on the right and select "Create Release".
- Set the title to `Release 2.4.0`.
- In the release notes body put the **Summary** and **Changelog**.
- Use the "+ Auto-generate release notes" button to add details from included PRs.
- [x] Make sure the new release shows up on [crates.io] and that the docs are built correctly on [docs.rs].
- [x] Announce the release, using the **Summary**, on Discord, Twitter and Mastodon.
- [x] Celebrate 🎉

[Semantic Versioning]: https://semver.org/
[crates.io]: https://crates.io/crates/bdk
[docs.rs]: https://docs.rs/bdk/latest/bdk
["keep a changelog"]: https://keepachangelog.com/en/1.0.0/