Modify configuration online can't work if the cluster is upgrade from v3.0.x

[Win-Man]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. Deploy a cluster in v3.0.5
2. Upgrade cluster from v3.0.5 to v4.0.7
3. Execute set config tikv raftstore.sync-log=false; in TiDB

MySQL [(none)]>  set config tikv raftstore.sync-log=false;
Query OK, 0 rows affected, 1 warning (0.01 sec)

MySQL [(none)]> show warnings;
+---------+------+---------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                               |
+---------+------+---------------------------------------------------------------------------------------+
| Warning | 1105 | Post http://0.0.0.0:30185/config: dial tcp 0.0.0.0:30185: connect: connection refused |
+---------+------+---------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

MySQL [(none)]> show config where name = 'raftstore.sync-log';
Empty set, 1 warning (0.01 sec)

MySQL [(none)]> show config where name like 'sync';
Empty set, 1 warning (0.01 sec)

MySQL [(none)]> show warnings;
+---------+------+--------------------------------------------------------------------------------------+
| Level   | Code | Message                                                                              |
+---------+------+--------------------------------------------------------------------------------------+
| Warning | 1105 | Get http://0.0.0.0:30185/config: dial tcp 0.0.0.0:30185: connect: connection refused |
+---------+------+--------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

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

Modify tikv's configuration online success.

3. What did you see instead (Required)

Modify tikv's configuration online failed.

4. What is your TiDB version? (Required)

MySQL [(none)]> select tidb_version()\G
*************************** 1. row ***************************
tidb_version(): Release Version: v4.0.6
Edition: Community
Git Commit Hash: 51d365fc45fdfc039eb204a96268c5bd1c55075f
Git Branch: heads/refs/tags/v4.0.6
UTC Build Time: 2020-09-15 09:50:30
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false
1 row in set (0.00 sec)

[lhy1024]

/label component/tikv