[CI] InternalAggregationsTests testSerialization reproducibly failing

[mark-vieira]

org.elasticsearch.search.aggregations.InternalAggregationsTests » testSerialization has failed 7 times today and hasn't failed otherwise in the past month, so it's likely something is busted here.

Here's a build failure for which the reproduction line reproduces locally for me: https://gradle-enterprise.elastic.co/s/jrqqlb4u4unv2/tests/nfxodyych4uas-dbj4whiivduzy

[elasticmachine]

Pinging @elastic/es-search (:Search/Search)

[cbuescher]

Looks like yet again a subtle time zone glitch, only triggered when the serialization is done on a version <7.0. The time zone of the rounding inside an InternalDateHistogram is set to "Z" (Zulu time) which seems to be equivalent to UTC. For versions before 7.0 we don't directly write the zone ID but convert to a string representation which in this case gives "UTC". This is deserialized to a "UTC" time zone which is equivalent but fails our strict equality checks.
I'll mute the test for now and work on a fix.

[cbuescher]

Should be closed by #50845