With zookeeper 3.5.x now marked as stable and 3.5 being reccomend for
the production branch (see https://zookeeper.apache.org/releases.html),
this moves pulsar up to 3.5.6.

Pulsar previously had 3.5.x support but was downgraded due to concerns
of 3.5 still being a "beta", so this is a fairly simple change

This also uses 3.5.6 as it has a fix for an issue where upgrades fail if
there is no snapshot files

Fixes apache#4448