`TIMESTAMP` shouldn't shift timezone

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

### 2. What did you expect to see? (Required)

```

mysql> SELECT TIMESTAMP('2023-10-10T12:00:00Z');
+-----------------------------------+
| TIMESTAMP('2023-10-10T12:00:00Z') |
+-----------------------------------+
| 2023-10-10 12:00:00 |
+-----------------------------------+
1 row in set, 1 warning (0.00 sec)

mysql> select @@time_zone;
+-------------+
| @@time_zone |
+-------------+
| +08:00 |
+-------------+
1 row in set (0.04 sec)
```

### 3. What did you see instead (Required)

```
mysql> SELECT TIMESTAMP('2023-10-10T12:00:00Z');
+-----------------------------------+
| TIMESTAMP('2023-10-10T12:00:00Z') |
+-----------------------------------+
| 2023-10-10 20:00:00 |
+-----------------------------------+
1 row in set (0.00 sec)

mysql> select @@time_zone;
+-------------+
| @@time_zone |
+-------------+
| SYSTEM |
+-------------+
1 row in set (0.00 sec)
```

SYSTEM is utc+8.

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