Make time series start and end time settings required settings

[martijnvg]

The index.time_series.start_time and index.time_series.end_time settings are required for tsdb backing indices of a data stream. However for standalone tsdb indices these settings are optional with somewhat extreme default values, 01-01--9999 and 31-12-9999 respectively. I suspect this was done for ease of testing tsdb functionality in a standalone index.

For tsdb data streams it shouldn't currently be possible to get into a situation that a backing index has no index.time_series.start_time or index.time_series.end_time settings. However in the past it was possible due a bug and I don't think it is watertight. I think we need also need to make index.time_series.start_time and index.time_series.end_time settings required when defining tsdb indices. This would make the tsdb data stream validation easier and less error prone.

Found as part of investigating #98792

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Pinging @elastic/es-analytics-geo (Team:Analytics)

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