settings: Rename 'zeta' to 'zed' (#23174)

Old:

```settings.json
{
  "features": {
    "inline_completion_provider": "zeta"
  }
}
```

New & cool:

```settings.json
{
  "features": {
    "inline_completion_provider": "zed"
  }
}
```

Release Notes:

- N/A

crates/language/src/language_settings.rs

@@ -203,7 +203,7 @@ pub enum InlineCompletionProvider {
     #[default]
     Copilot,
     Supermaven,
-    Zeta,
+    Zed,
 }
 
 /// The settings for inline completions, such as [GitHub Copilot](https://github.com/features/copilot)