tidb_enable_noop_variables cannot be set to WARN

[djshow832]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

This is introduced by #35496.

mysql> set global tidb_enable_noop_variables='warn';
Query OK, 0 rows affected (0.00 sec)

mysql> show variables like 'tidb_enable_noop_variables';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| tidb_enable_noop_variables | OFF   |
+----------------------------+-------+
1 row in set (0.00 sec)

mysql> select * from information_schema.variables_info where variable_name='tidb_enable_noop_variables';
+----------------------------+-----------------+---------------+---------------+-----------+-----------+-----------------+---------+
| VARIABLE_NAME              | VARIABLES_SCOPE | DEFAULT_VALUE | CURRENT_VALUE | MIN_VALUE | MAX_VALUE | POSSIBLE_VALUES | IS_NOOP |
+----------------------------+-----------------+---------------+---------------+-----------+-----------+-----------------+---------+
| tidb_enable_noop_variables | GLOBAL          | ON            | OFF           |      NULL |      NULL | OFF,ON,WARN     | NO      |
+----------------------------+-----------------+---------------+---------------+-----------+-----------+-----------------+---------+
1 row in set (0.01 sec)

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

set global tidb_enable_noop_variables='warn'; should report a warning or error.
POSSIBLE_VALUES should not include warn.

3. What did you see instead (Required)

possible_values includes warn, but it cannot be set to warn.

4. What is your TiDB version? (Required)

master.