Currently, DateHistogramAggregationBuilder#rewriteTimeZone uses the aggregation
date math parser and time zone to check whether all values in a read have the
same timezone to speed up computation. However, the upper and lower bounds to
check are retrieved as longs in epoch_millis, so they don't need to get parsed
using a time zone or a parser other than "epoch_millis". This changes this
behaviour that was causing problems when the field type mapping was specifying
only "epoch_millis" as a format but a different timezone than UTC was used.

Closes elastic#31392