Update CHANGELOG describing nextcloud chart update

CHANGELOG.md

+## Unreleased
+
+* Update NC chart to 2.9.0
+  - This in turn updates the mariadb dependency chart from 7.10 to 9.6, which
+    brings along some changes to helm value names: see [their upgrade
+    notes](https://github.com/bitnami/charts/tree/master/bitnami/mariadb/#to-800).
+  - Also, this requires Helm version 3.7 or higher to install (or upgrade).
+
 ## [0.3.3] - 2021-09-02
 
 * Add startupProbe for ONLYOFFICE