[chore][exporter/elasticsearch] docs: otel mapping mode requires ES 8.12+ (#38427)

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#### Description

Update docs to mention that otel mapping mode requires Elasticsearch
8.12+ because of require_data_stream.

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#### Link to tracking issue
Fixes #37283

Author: carsonip

exporter/elasticsearchexporter/README.md

@@ -177,6 +177,8 @@ See below for a description of each mapping mode.
 
 #### OTel mapping mode
 
+Requires Elasticsearch 8.12 or above.
+
 In `otel` mapping mode, the Elasticsearch Exporter stores documents in Elastic's preferred
 "OTel-native" schema. In this mapping mode, documents use the original attribute names and
 closely follows the event structure from the OTLP events.