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
```

首先复现将 retentionPolicy 设置为 "7d" 的 ObjectStore 配置，以及使用 Barman Cloud 插件的 ScheduledBackup。比较 WAL 和基础备份的 pruning 行为，包括 Hetzner 的 S3 兼容端点；当早于配置的 retention 周期的基础备份被 pruning，且不影响 WAL retention 时，即表示完成。

由索引模型根据 Issue 内容生成。