Aggregations: Fixes pre and post offset serialisation for histogram aggs

Changes the serialisation of pre and post offset to use Long instead of VLong so that negative values are supported.  This actually only showed up in the case where minDocCount=0 as the rounding is only serialised in this case.

Closes #7312
It think this is problematic since this forces LocalTransport all the time even if we run with network tests. I think you should add a setting to `AssertingLocalTransport` that allows you to set the min version? then you can just pass the min version here instead of the `TransportModule.TRANSPORT_TYPE_KEY` I hope that makes sense