[SLO][8.11] Add date_formats to SLI ingest pipeline template (#172377)

## Summary

This PR fixes #172372 by adding the `date_formats` attribute to the
`date_index_name` pipeline step for the SLI ingest pipeline that every
SLO runs through. This PR is only for 8.11, the fix for main will be
included with: #172224

x-pack/plugins/observability/server/assets/ingest_templates/slo_pipeline_template.ts

@@ -16,6 +16,7 @@ export const getSLOPipelineTemplate = (id: string, indexNamePrefix: string) => (
         field: '@timestamp',
         index_name_prefix: indexNamePrefix,
         date_rounding: 'M',
+        date_formats: ['UNIX_MS', 'ISO8601', "yyyy-MM-dd'T'HH:mm:ss.SSSXX"],
       },
     },
   ],