Change log for ZSCALER_DNS

Date Changes
2025-05-14 - Backward compatibility code behaved erratically because two fields were getting merged within a single filter. Modified the code to handle this scenario.
2025-04-30 - Promoted ZSCALER_DNS Premium parser to default. You can see full details in the parser configuration page - https://cloud.google.com/chronicle/docs/ingestion/default-parsers/ingest-zscaler-logs