doc: update Velero config with new restic flag

[nvanthao]

There's a breaking change from Velero 1.10

https://github.com/vmware-tanzu/velero/blob/main/changelogs/CHANGELOG-1.10.md#breaking-changes

restic daemonset is renamed to node-agent resticRepository CR is renamed to backupRepository velero restic repo command is renamed to velero repo velero-restic-credentials secret is renamed to velero-repo-credentials default-volumes-to-restic parameter is renamed to default-volumes-to-fs-backup restic-timeout parameter is renamed to fs-backup-timeout default-restic-prune-frequency parameter is renamed to default-repo-maintain-frequency

Our default advanced spec has --default-restic-prune-frequency=12h which will cause the installation to be failed with pod error An error occurred: unknown flag: --default-restic-prune-frequency

I have updated the doc with the new flag.

[github-actions]

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