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/zed/src/zed/inline_completion_registry.rs

@@ -164,7 +164,7 @@ fn assign_inline_completion_provider(
                 editor.set_inline_completion_provider(Some(provider), cx);
             }
         }
-        language::language_settings::InlineCompletionProvider::Zeta => {
+        language::language_settings::InlineCompletionProvider::Zed => {
             if cx.has_flag::<ZetaFeatureFlag>()
                 || (cfg!(debug_assertions) && client.status().borrow().is_connected())
             {