Until a couple of hours ago we expected the position_offset_gap to default
to 0 in 2.0 and 100 in 2.1. We decided it was worth backporting that new
default to 2.0. So now that its backported we need to teach 2.1 that 2.0
also defaults to 100.
Closes#7268
This move the `murmur3` field to the `mapper-murmur3` plugin and fixes its
defaults so that values will not be indexed by default, as the only purpose
of this field is to speed up `cardinality` aggregations on high-cardinality
string fields, which only requires doc values.
I also removed the `rehash` option from the `cardinality` aggregation as it
doesn't bring much value (rehashing is cheap) and allowed to remove the
coupling between the `cardinality` aggregation and the `murmur3` field.
Close#12874