Invalid value: "object": BackupSpec is immutable once set

Hi, Team !

I'm doing migrations from Barman Native to Barman Plugin (0.7.0) using the same set of commands.

I have two setups:
1) CNPG 1.26.1
2) CNPG 1.27.0

My configs are:

```
tee $cluster_name.yaml.NATIVE << EOF
apiVersion: postgresql.cnpg.io/v1
kind: Cluster
metadata:
name: $cluster_name
namespace: $oname-system
spec:
instances: 3
imageName: $cnpg_image_name
storage:
size: 1Gi
backup:
retentionPolicy: "1d"
barmanObjectStore:
destinationPath: s3://cluster-backups/
endpointURL: http://minio-service:9000
s3Credentials:
accessKeyId:
name: minio-creds
key: ACCESS_KEY_ID
secretAccessKey:
name: minio-creds
key: ACCESS_SECRET_KEY
wal:
compression: gzip
EOF
```

```
tee barman-backup.yaml.NATIVE << EOF
apiVersion: postgresql.cnpg.io/v1
kind: Backup
metadata:
name: barman-backup
namespace: $oname-system
spec:
cluster:
name: $cluster_name
target: prefer-standby
EOF
```

```
tee minio-store.yaml << EOF
apiVersion: barmancloud.cnpg.io/v1
kind: ObjectStore
metadata:
name: minio-store
namespace: $oname-system
spec:
configuration:
destinationPath: s3://cluster-backups/
endpointURL: http://minio-service:9000
s3Credentials:
accessKeyId:
name: minio-creds
key: ACCESS_KEY_ID
secretAccessKey:
name: minio-creds
key: ACCESS_SECRET_KEY
wal:
compression: gzip
retentionPolicy: "1d"
EOF
```

```
tee $cluster_name.yaml.PLUGIN << EOF
apiVersion: postgresql.cnpg.io/v1
kind: Cluster
metadata:
name: $cluster_name
namespace: $oname-system
spec:
instances: 3
imagePullPolicy: Always
imageName: $cnpg_image_name
storage:
size: 1Gi
plugins:
- name: barman-cloud.cloudnative-pg.io
isWALArchiver: true
parameters:
barmanObjectName: minio-store
EOF
```

```
tee barman-backup.yaml.PLUGIN << EOF
apiVersion: postgresql.cnpg.io/v1
kind: Backup
metadata:
name: barman-backup
namespace: $oname-system
spec:
cluster:
name: $cluster_name
method: plugin
pluginConfiguration:
name: barman-cloud.cloudnative-pg.io
target: prefer-standby
EOF
```

The set of commands is:
a) `kubectl apply --server-side -f $oname-$cnpg_full_version.yaml`
b) `kubectl apply -f minio.yaml`
c) `kubectl apply -f $cluster_name.yaml.NATIVE`
d) `kubectl apply -f barman-backup.yaml.NATIVE`
e) `kubectl apply -f cert-manager.yaml`
f) `kubectl apply -f manifest.yaml`
g) `kubectl apply -f minio-store.yaml`
h) `kubectl apply -f $cluster_name.yaml.PLUGIN`
i) `kubectl apply -f barman-backup.yaml.PLUGIN`

This set of commands works fine for the setup 1) (CNPG 1.26.1).
However for the setup 2) (CNPG 1.27.0) I have the following message for the step i)

```
[alexey@wsl2 ~]$ kubectl apply -f barman-backup.yaml.PLUGIN
The Backup "barman-backup" is invalid: spec: Invalid value: "object": BackupSpec is immutable once set
```

What's wrong with it ?

Commencez par les manifestes CNPG 1.26.1 et 1.27.0, en particulier barman-backup.yaml.NATIVE et barman-backup.yaml.PLUGIN, et reproduisez step i avec kubectl apply. Suivez l’endroit où l’immuabilité de BackupSpec est validée et comparez la séquence de migration native-to-plugin. Le travail est considéré comme terminé lorsque la séquence de commandes documentée s’applique correctement sur CNPG 1.27.0, ou lorsque l’incompatibilité et la configuration requise sont clairement documentées.

Rédigé par le modèle d'indexation à partir du texte de l'issue.