Autocast Keyword to Date in Date Nanos comparisons

https://github.com/elastic/elasticsearch/issues/116281 added support for comparing nanosecond dates and millisecond dates, but it didn't extend the keyword to date auto-promotion logic to that case.  We would like that to work, so that expressions like `nanos > "2023-01-01"` are valid ES|QL.