[Backport master] Fix failing transform tests and support dates_as_epoch_millis #5212
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Tests were failing due to a change in the transform API which now
prefers ISO dates, rather than epoch milliseconds for dates. This
resulted in a serialisation failure.
This commit addresses that by conditionally producing the requests
based on the version of ES being tested, so that a property of the
appropriate type is used for deserialisation of the preview.
This also adds support for the now dates_as_epoch_millis setting which
allows the original epoch millisecond format to be returned for ES 7.11
requests, when the option is true.
(cherry picked from commit 7994d6f)
Backport 7994d6f from #5209