[Dashboard] Bumped default REPORTER_UPDATE_INTERVAL_MS to 5s

[alexeykudinkin]

Why are these changes needed?

To reduce the pressure induced by

• Relatively high-frequency of node-stats updates originated from Reporter Agent
• Using JSON instead of Proto for payloads encoding

Related issue number

While this change is not addressing #45191, it aims at reducing the pressure on the Dashboard Head process by reducing the frequency of updates

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• Testing Strategy
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