Reduce librdkafka logs
Issue
librdkafka generates warning logs, mostly due to configuration warnings when applying consumer settings to producers, and viceversa.
Since we create large numbers of consumers, these configuration warnings clutter our logs way too much.
Description of work
Demote CONFWARN librdkafka logs to DEBUG level, so they can easily be filtered out in Graylog
Checklist
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Changes have been documented in changes.md
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Documentation has been updated in documentation
directory