Add Grok pattern for ISO 8601 datestamp

[philrz]

What's Changing

Adding a grok pattern for a date of the format 2024-08-15.

Why

A user went looking for such a pattern in a recent community Slack thread and indeed one didn't exist in our set of included patterns.

Details

While I showed the user how to define/use one immediately via Grok's optional third definitions argument, this pattern seems common enough that it's worth adding to our included base set.

I've done a fair amount of timestamp-related research lately while putting together the write-up in #5221, so I put a little extra throught into the name of this pattern, e.g., should it be DATESTAMP_ISO8601 or something else like DATESTAMP_RFC3339. However, I went with the former because:

1. A date of this format is the top entry at https://en.wikipedia.org/wiki/ISO_8601, and,
2. RFC3339 is considered a "profile" of ISO 8601, making ISO 8601 the "wider umbrella" thing to reference

[mattnibs]

I was skeptical about adding this since it's not included in grok-patterns for logstash, but then again we do have extra base rules extended beyond logstash and being that this is a subset of TIMESTAMP_ISO8601 so I don't think it hurts to add this as well.