chore: datafusion is now a top-level Apache project (ibis-project#9024)

## Description of changes

Apache Arrow DataFusion -> Apache Datafusion. the only places this was
written include:

1. README (updated here)
2. portability.excalidraw (no longer used, deleted along w/ the .png)
3. release_notes_generated.qmd (historical, accurate at the time so
unchanged)

## Issues closed

README.md

@@ -135,7 +135,7 @@ This allows you to combine the flexibility of Python with the scale and performa
 
 Ibis supports 20+ backends:
 
-- [Apache Arrow DataFusion](https://ibis-project.org/backends/datafusion/)
+- [Apache DataFusion](https://ibis-project.org/backends/datafusion/)
 - [Apache Druid](https://ibis-project.org/backends/druid/)
 - [Apache Flink](https://ibis-project.org/backends/flink)
 - [Apache Impala](https://ibis-project.org/backends/impala/)