Release v3.21.0

This release for the backend features improvements to the control api, and improvements to our ability to run automated tests.

Features

control-api: Create retry endpoint

forwarder: Create test-error sink to help with QA and simulating delivery errors

Remove kafka logging allocations to improve memory performance

ingestor: add alpha-numeric validation to custom event write key

forwarder: Use Control API retry endpoint

terraform+helm: Re-enable nr_ metric prefix & add non-critical retry lag alert

retrier: Improve performance; ack all dropped messages

terraform: Add pubsub resources for staging test-error forwarder

helm: Remove deprecated retry config from forwarder

Let staging use default kafka settings to mimic prod environments