[EuiCodeBlock] Update the annotation to be `leftCenter`

**Problem Statement**

If there is a horizontal space available, we don't want annotations to overlap the code.

**Proposed Solution**

| Current ([docs](https://eui.elastic.co/docs/components/display/code/#line-numbers)) | Desired |
|---------|---------|
| | |

If the space allows, we want the annotation to be `leftCenter`. If there is not enough horizontal space, we want it to default to `downLeft`.

**Use Case**

This was raised by an internal user, AutoOps.

**Value / Impact**

The value is **low** and the impact is **low**. It wouldn't be a breaking change.

**Urgency**

There is **no urgency**. It doesn't block any feature.

**Do alternatives or workarounds exist?**

There are no workarounds available.