@@global.character_set_connection seems take no effect for new connections

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```
mysql> set @@global.character_set_connection="latin1";
```

Then schedule a new connection using MySQL client:

```
$ mysql --comments --host 127.0.0.1 -uroot -P4000
mysql> select @@global.character_set_connection, @@character_set_connection;
```

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

I use a mysql client with version `8.0.33` to do this test

```
$ mysql --version
mysql Ver 8.0.33 for macos13.3 on arm64 (Homebrew)
```

In mysql 8.0.33, the new connection has the same `@@character_set_connection` with global settings:

```
$ mysql --comments --host 127.0.0.1 -uroot -P3306
mysql> select @@global.character_set_connection, @@character_set_connection;
+-----------------------------------+----------------------------+
| @@global.character_set_connection | @@character_set_connection |
+-----------------------------------+----------------------------+
| latin1 | latin1 |
+-----------------------------------+----------------------------+
1 row in set (0.00 sec)

mysql> select version();
+-----------+
| version() |
+-----------+
| 8.0.33 |
+-----------+
1 row in set (0.01 sec)
```

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

But for TiDB, `@@global.character_set_connection` does not take effect:

```
> mysql --comments --host 127.0.0.1 -uroot -P4001
mysql> select @@global.character_set_connection, @@character_set_connection;
+-----------------------------------+----------------------------+
| @@global.character_set_connection | @@character_set_connection |
+-----------------------------------+----------------------------+
| latin1 | utf8mb4 |
+-----------------------------------+----------------------------+
1 row in set (0.00 sec)
```

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

master