Helm upgrade of nextcloud's redis fails

We changed some fundamental things about nextcloud's redis subchart (disabling persistence), see #1498 (closed). This fails when upgrading existing clusters, because fields of the redis StatefulSet would be changed that are immutable. We should be able to solve this by adding the uninstall upgrade remediation strategy to the nextcloud HelmRelease. This will instruct flux to reinstall nextcloud, circumventing the immutability.