base backup are not pruned (even though retention is set)

hey, i’ve encountered a problem - retention policy works for wals, but not for base backups. Base backups are not pruned at all. Im using hetzner s3 if it matters, wals retention works fine

```
apiVersion: barmancloud.cnpg.io/v1
kind: ObjectStore
metadata:
name: main-db-object-store
namespace: main-db
spec:
retentionPolicy: "7d"

# needed for non-AWS S3-compatible object storages
instanceSidecarConfiguration:
env:
- name: AWS_REQUEST_CHECKSUM_CALCULATION
value: when_required
- name: AWS_RESPONSE_CHECKSUM_VALIDATION
value: when_required
- name: AWS_MAX_ATTEMPTS
value: "10"

configuration:
destinationPath: "...."
endpointURL: "https://hel1.your-objectstorage.com/"
s3Credentials:
accessKeyId:
name: hetzner-s3-creds
key: ACCESS_KEY_ID
secretAccessKey:
name: hetzner-s3-creds
key: ACCESS_SECRET_KEY
wal:
compression: gzip
maxParallel: 4
archiveAdditionalCommandArgs:
- "--read-timeout=1200"
data:
compression: gzip
jobs: 1
additionalCommandArgs:
- "--read-timeout=3600"
- "--min-chunk-size=512MB"
---
apiVersion: postgresql.cnpg.io/v1
kind: ScheduledBackup
metadata:
name: main-db-daily-backup
namespace: main-db
spec:
schedule: "0 0 2 * * *"
backupOwnerReference: self
cluster:
name: main-db
method: plugin
pluginConfiguration:
name: barman-cloud.cloudnative-pg.io
```

Start by reproducing the ObjectStore configuration with retentionPolicy set to "7d" and the ScheduledBackup using the Barman Cloud plugin. Compare pruning behavior for WALs and base backups, including the Hetzner S3-compatible endpoint; done means base backups older than the configured retention period are pruned without breaking WAL retention.

索引モデルが issue の本文から書いたものです。