Update flex_parse_date() to extract milliseconds

## What?

`flex_parse_date()` should include milliseconds resolution when the source value includes milliseconds; another option would be to add a date parsing function that accepts a list of patterns and processes each one.

## Why?

Seconds resolution may not precise enough when investigating security issues, and we can't always predict which format date/time values are configured with.

## Your Environment

* Graylog Version:
* OpenSearch Version:
* MongoDB Version:
* Operating System:
* Browser version: