Swimlanes - Use VirtualList to improve loading experience [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

Introduces swimlanes_buffered_rendering feature flag.

Uses VirtualList on Epic Swimlanes to improve loading experience when there's a lot of epics.

Issue #324736 (closed)

Screenshots (strongly suggested)

No visual changes.

Does this MR meet the acceptance criteria?

Conformity

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Security

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