learning-the-basics: Migrate system updates from AppCenter to System Settings

### What Happened?

We migrated system updates from AppCenter to System Settings in OS 7, but Learning the Basics remains old and says "AppCenter includes updates for apps and the operating system itself under the Installed tab."

### Steps to Reproduce

1. Visit https://elementary.io/en/docs/learning-the-basics#updating

### Expected Behavior

We should update the doc to mention that system updates are available in System Settings while app updates are still available on AppCenter.

### OS Version

8.x (Circe)

### OS Architecture

arm64 (on Raspberry Pi, Apple Silicon, etc.)

### Session Type

Secure Session (Wayland, This is the default)

### Software Version

Compiled from git

### Log Output

```shell

```

### Hardware Info

_No response_