[Task]: Remove Flink 1.15 and cleanup

[kennknowles]

What needs to happen?

Currently we have five versions of Flink support, when policy is three. Obviously we don't have to be rigid about policy. But we can probably drop 1.15 immediately and then continue from there.

Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

Issue Components

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• [X] Component: Flink Runner
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