Configure the CloudNativePG primary update method in the Helm chart

The Helm chart now exposes postgresql.cnpg.primaryUpdateMethod, so you can choose how CloudNativePG rolls the primary during updates. It still defaults to switchover to keep existing behavior, but you can set it to restart to avoid a switchover deadlock when moving backups to the Barman Cloud Plugin. The Tuist production deployment uses this setting for its cutover.