In TiDB 7.1.6, using Placement Rules in SQL to move the entire database to cold storage does not take effect.

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)
1、经测试TiDB 7.1.6版本，不支持设置库级别placement rule in sql
```sql
ALTER database sbtest1 PLACEMENT POLICY=storeon_ssdslow;
ALTER database data_test PLACEMENT POLICY=storeon_ssdslow;

root@127.0.0.1 11:06:40 [sbtest1]> show placement;
+------------------------+-------------------------------------+------------------+
| Target | Placement | Scheduling_State |
+------------------------+-------------------------------------+------------------+
| POLICY storeon_fastssd | CONSTRAINTS="[+disk_type=fast_ssd]" | NULL |
| POLICY storeon_ssdslow | CONSTRAINTS="[+disk_type=slow_ssd]" | NULL |
| DATABASE data_test | CONSTRAINTS="[+disk_type=slow_ssd]" | SCHEDULED |
| DATABASE sbtest1 | CONSTRAINTS="[+disk_type=slow_ssd]" | SCHEDULED |
+------------------------+-------------------------------------+------------------+
```

### 2. What did you expect to see? (Required)
data/region move to "storeon_ssdslow" store.
### 3. What did you see instead (Required)
none data/region moved to "storeon_ssdslow" store.

### 4. What is your TiDB version? (Required)
v7.1.6