Use `date_nanos` as timestamp format when `index.mode` is `logs`

[salvatore-campagna]

Description

As part of the settings we would like to apply by default when index.mode is set to logs we are evaluating if we should also use date_nanos as the default for the timestamp field. Many integrations use such format including integrations through OTel. Introducing it before making the new index mode GA would allow us to deal with backward compatibility issues while the new index mode is still in tech preview.

This needs to be do keeping in mind backward compatibility.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[kkrik-es]

@not-napoleon fyi, this is related to #109352.