The field `max_user_connections` should all lowercase letters

## Enhancement

In mysql
```
mysql-9.2.0> SHOW FIELDS FROM mysql.user LIKE 'max\_%';
+----------------------+--------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------------+--------------+------+-----+---------+-------+
| max_questions | int unsigned | NO | | 0 | |
| max_updates | int unsigned | NO | | 0 | |
| max_connections | int unsigned | NO | | 0 | |
| max_user_connections | int unsigned | NO | | 0 | |
+----------------------+--------------+------+-----+---------+-------+
4 rows in set (0.00 sec)
```

But in TiDB

```
mysql> SHOW FIELDS FROM mysql.user LIKE 'max\_%';
+----------------------+--------------+------+------+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------------------+--------------+------+------+---------+-------+
| Max_user_connections | int unsigned | NO | | 0 | |
+----------------------+--------------+------+------+---------+-------+
1 row in set (0.01 sec)
```

The field `Max_user_connections` should all lowercase letters.