[Task]: Update roadmap for Spark runner

[mosche]

What needs to happen?

The Spark related roadmap is far outdated and needs some attention.

• Spark 3 is fully supported.
• Spark dataset runner is on it's way.

Issue Priority

Priority: 2

Issue Component

Component: runner-spark

[rszper]

I think this refers to this page: https://beam.apache.org/roadmap/spark-runner/