Update the Roadmap.md

[HaoYang670]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

Having Ballista as part of the DataFusion codebase helps ensure that DataFusion remains suitable for distributed compute. 
...

Should we update this part as Ballista has been a top-level repo ?

link: https://github.com/apache/arrow-datafusion/blob/master/docs/source/contributor-guide/roadmap.md?plain=1#L100-L102

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[HaoYang670]

cc @andygrove.

[alamb]

It would be great to update the roadmap

[jackwener]

We also can consider other part, some content is outdate.