Release DataFusion `56.0.0` (Oct 2026)

### Is your feature request related to a problem or challenge?
- Part of https://github.com/apache/datafusion/issues/19783

Previous release is https://github.com/apache/datafusion/issues/22393 (Jul / Aug 2026) so next major release would be approximately Sep / Oct 2026.

Steps:
- [ ] Create a `branch-56` branch
- [ ] Update version and changelog
- [ ] Test with DataFusion Python
- [ ] Test with DataFusion Comet
- [ ] Test with DataFusion Ballista
- [ ] Test with delta.rs
- [ ] Test vortex
- [ ] Test with iceberg-rust
- [ ] Test with LakeSail
- [ ] Test with parquet viewer
- [ ] Test with datafusion-materialized-views
- [ ] Voting Thread
- [ ] Publish to crates.io
- [ ] Create ticket for next release: https://github.com/apache/datafusion/issues/24463
- [ ] Create blog post

## Features to mention in the blog (if they make it)

- [ ] `ASOF` joins from @Xuanwo : https://github.com/apache/datafusion/issues/318
- [ ] https://github.com/apache/datafusion/issues/24608

## Bugs that should be fixed

- [ ] TBD

# Community Wishlist

- [ ] TBD