chore: update bdk-ffi to v2.3.1

[github-actions]

A new upstream bdk-ffi release is available.

• Current pinned tag in native/Cargo.toml: v2.3.0
• Latest upstream release: v2.3.1
• Release notes: https://github.com/bitcoindevkit/bdk-ffi/releases/tag/v2.3.1

Suggested follow-up:

1. Update the bdk-ffi tag in native/Cargo.toml.
2. Regenerate bindings with ./scripts/generate_bindings.sh.
3. Run dart test (and CI) before merging.