Detection of Instance Type is incorrect for Postgres 18

### Bug Description

I tried to use the proxy with a cloud sql instance using Postgres 18. It seems like the proxy is detecting it as a MySQL instance trying to connect on 3337, making it unusable.

```
2025/10/27 07:12:52 proxy server error: dial error: failed to dial (connection name = "#########"): dial tcp ########:3307: connect: connection refused
```

It should be compatible with the versions offered in cloud sql, especially given that we can't rollback an upgrade in CLoud SQL.

Thank you

### Example code (or command)

```
// paste your code or command here
```

### Stacktrace

```bash

```

### Steps to reproduce?

1. ?
2. ?
3. ?
...

### Environment

1. OS type and version:
2. Cloud SQL Proxy version (`./cloud-sql-proxy --version`):
3. Proxy invocation command (for example, `./cloud-sql-proxy --port 5432 INSTANCE_CONNECTION_NAME`):

### Additional Details

_No response_