[ML] Use event.timezone instead of beat.timezone in file upload (#58447)

This is because beat.timezone was renamed to event.timezone in
elastic/beats#9458

The corresponding file structure finder change is
elastic/elasticsearch#52720

x-pack/legacy/plugins/ml/public/application/datavisualizer/file_based/components/import_view/importer/importer.js

@@ -148,17 +148,17 @@ function populateFailures(error, failures, chunkCount) {
   }
 }
 
-// The file structure endpoint sets the timezone to be {{ beat.timezone }}
+// The file structure endpoint sets the timezone to be {{ event.timezone }}
 // as that's the variable Filebeat would send the client timezone in.
 // In this data import function the UI is effectively performing the role of Filebeat,
 // i.e. doing basic parsing, processing and conversion to JSON before forwarding to the ingest pipeline.
 // But it's not sending every single field that Filebeat would add, so the ingest pipeline
-// cannot look for a beat.timezone variable in each input record.
-// Therefore we need to replace {{ beat.timezone }} with the actual browser timezone
+// cannot look for a event.timezone variable in each input record.
+// Therefore we need to replace {{ event.timezone }} with the actual browser timezone
 function updatePipelineTimezone(ingestPipeline) {
   if (ingestPipeline !== undefined && ingestPipeline.processors && ingestPipeline.processors) {
     const dateProcessor = ingestPipeline.processors.find(
-      p => p.date !== undefined && p.date.timezone === '{{ beat.timezone }}'
+      p => p.date !== undefined && p.date.timezone === '{{ event.timezone }}'
     );
 
     if (dateProcessor) {