currently only Splunk Enterprise 9.x is officially supported by Splunk. So you should update to this. Just select which version is enough mature and don't contains any security issues. Usually I said that select something else than x.0.z where z < 4 or x.y.z where z < 3. Also you should wait at least couple of days/weeks after a new GA version has published before try it on production.
There are compatibility matrix which told which UF you could use with splunk enterprise / cloud.
