set sql_mode=1 process is different with mysql

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

SET sql_mode = 1;

### 2. What did you expect to see? (Required)
in Mysql 8.0.29
```sql
mysql> set sql_mode=1;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like 'sql_mode';
+---------------+---------------+
| Variable_name | Value |
+---------------+---------------+
| sql_mode | REAL_AS_FLOAT |
+---------------+---------------+
1 row in set (0.01 sec)
```
### 3. What did you see instead (Required)
in tidb v6.1.0
```sql
mysql> SET sql_mode = 1;
ERROR 1105 (HY000): ERROR 1231 (42000): Variable 'sql_mode' can't be set to the value of '1'
```
### 4. What is your TiDB version? (Required)

Git Commit Hash: 1a89decdb192cbdce6a7b0020d71128bc964d30f
Git Branch: heads/refs/tags/v6.1.0
UTC Build Time: 2022-06-05 05:15:11