What does this PR do?

Update multi-alert group limit for Event Monitor.
According to the event migration document, limit is up to 1000 instead of 100.

Motivation

https://docs.datadoghq.com/events/guides/migrating_to_new_events_features/#you-can-group-by-only-up-to-4-facets

One facet results in a limit of 1000 top values.
Two facets results in a limit of 30 top values per facet (at most 900 groups)
Three facets results in a limit of 10 top values per facet (at most 1000 groups)
Four facets results in a limit of 5 top values per group (at most 625 groups)

Additional Notes

Reviewer checklist

• Review the changed files.
• Review the URLs listed in the Preview section.
• Check images for PII
• Review any mentions of "Contact Datadog support" for internal support documentation.