Update field-mapping.asciidoc that Epoch format is not supported as dynamic date format

[kunisen]

Update field-mapping.asciidoc that Epoch format is not supported as dynamic date format per https://github.com/elastic/elasticsearch/blob/main/server/src/main/java/org/elasticsearch/index/mapper/RootObjectMapper.java#L430-L431

if (formatter.toString().startsWith("epoch_")) {
  throw new MapperParsingException("Epoch [" + formatter + "] is not supported as dynamic date format");
}

It's in the code but not documented, and this PR is to document it explicitly.

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[github-actions]

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[elasticsearchmachine]

@kunisen please enable the option "Allow edits and access to secrets by maintainers" on your PR. For more information, see the documentation.

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[kunisen]

cc @bojian-ben-li

[abdonpijpelink]

Thanks @kunisen ! I've left a suggestion. What do you think?

[kunisen]

Thank you Abdon!! 🙇

[elasticsearchmachine]

💔 Backport failed

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 98338

[abdonpijpelink]

💚 All backports created successfully

Status	Branch	Result
✅	8.9

Questions ?

Please refer to the Backport tool documentation