Removed the existing `pre_zone` and `post_zone` option in `date_histogram` in favor of
the simpler `time_zone` option. Previously, specifying different values for these could
lead to confusing scenarios where ES would return bucket keys that are not UTC.
Now `time_zone` is the only option setting, the calculation of date buckets to take place in the
preferred time zone, but after rounding converting the bucket key values back to UTC.
Closes#9062Closes#9637
30fd70f07b