raise error when using tracking_field and search_api is not "search_after"

[okliv]

Bug Report Summary
When using tracking_field with logstash-input-elasticsearch and the default scroll API, Logstash sends the literal string :last_value instead of the resolved timestamp. This causes Elasticsearch to fail parsing with:

    failed to parse date field [:last_value] with format [strict_date_optional_time_nanos]
    caused_by: Text ':last_value' could not be parsed at index 0

Workaround
Use search_api => "search_after" instead of the default scroll.

System has to notify about it somehow. I've spent a lot of time while i figured it out.

[cla-checker-service]

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[robbavey]

Hi @okliv, would you mind signing the CLA please?

Thanks!