resourcegroup: max group name length is incompatible with MySQL

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

```sql
> tiup client
Connected with driver mysql (8.0.11-TiDB-v8.5.5)

> CREATE RESOURCE GROUP test_group_not_too_loooooooooooooooooooooong RU_PER_SEC = 1000;
error: mysql: 1105: resource group name too long
```

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

The resource group should be created successfully, as MySQL 8.0 allows resource group names up to 64 characters.

Reference: https://dev.mysql.com/doc/refman/8.0/en/identifier-length.html

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

TiDB rejects resource group names longer than 32 characters with the error resource group name too long.

This is because MaxGroupNameLength is set to 32 in pkg/ddl/resourcegroup/group.go, which is stricter than MySQL's limit of 64 characters, making TiDB incompatible with MySQL in this regard.

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

v8.5.5