Update the Spark Operator roadmap

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The `ROADMAP.md` in spark-operator is currently heavily out of date. It should either be updated or modified to be a bit more timeless (i.e. by referencing the projects planning concept and including links to the milestones or similar if that is being used).

### Additional context

https://github.com/cncf/toc/pull/2180/changes#r3477566496

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Start by reading ROADMAP.md in the spark-operator repository and review the linked CNCF TOC discussion for context. Update the roadmap or make it timeless by referencing the project's planning concept and linking relevant milestones, with the result documented in ROADMAP.md.

Written by the indexing model from the issue text.