[8.14] Data Streams: Add ecs@mappings disclaimer for index templates cloned on ES < 8.13 (backport #1174)

[mergify]

Motivation

Starting from version 8.13, Elasticsearch includes the ecs@mappings component templates in all the index templates used in Elastic Agent integrations.

As integrations adopt the ecs@mappings component template, index templates cloned following the "Tutorial: Customize data retention policies" may start having problems with mappings.

End users must update their cloned index templates by adding the ecs@mappings component template.

Change description

This PR added a note and instructions to the tutorial users about updating the cloned index template they created following this tutorial.

Additional Notes

• relates: Add reference to ecs@mappings for each index template kibana#174855

Reviewer checklist

• PR address a single concern.
• PR title and description are appropriately filled.
• Changes will be merged in main.

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This is an automatic backport of pull request #1174 done by [Mergify](https://mergify.com).

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