helm chart does not follow default file layout for engine-path

https://github.com/influxdata/helm-charts/blob/a42e6022de5cf0d6a020ae3c1a986396b5bc1122/charts/influxdb2/templates/statefulset.yaml#L103-L104

should be
```
value: {{ .Values.persistence.mountPath }}/engine
```
ref: https://docs.influxdata.com/influxdb/v2.5/reference/internals/file-system-layout/
Also mentioned #474

The consequence of this for our deployment was that we migrated to the helm-chart at some point, and then ended up with the previous metrics in ./engine/data and the new metrics in ./data. All of the old metrics were then not able to be queried.

Inspect charts/influxdb2/templates/statefulset.yaml at the referenced lines and compare the engine path with the InfluxDB file-system layout documentation. Confirm the rendered path uses the persistence mount path followed by /engine, and verify that existing metrics remain under the expected layout.

Written by the indexing model from the issue text.