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/inline_completion_button/src/inline_completion_button.rs

@@ -201,7 +201,7 @@ impl Render for InlineCompletionButton {
                 );
             }
 
-            InlineCompletionProvider::Zeta => {
+            InlineCompletionProvider::Zed => {
                 if !cx.has_flag::<ZetaFeatureFlag>() {
                     return div();
                 }