gpui: Rename rounded_sm to rounded_xs (#26221)

This PR renames the `rounded_sm` style method to `rounded_xs`.

This will allow us to add an additional step in the scale.

Release Notes:

- N/A

crates/gpui/examples/image_loading.rs

@@ -69,7 +69,7 @@ struct ImageLoadingExample {}
 
 impl ImageLoadingExample {
     fn loading_element() -> impl IntoElement {
-        div().size_full().flex_none().p_0p5().rounded_sm().child(
+        div().size_full().flex_none().p_0p5().rounded_xs().child(
             div().size_full().with_animation(
                 "loading-bg",
                 Animation::new(Duration::from_secs(3))
@@ -89,7 +89,7 @@ impl ImageLoadingExample {
                 .flex()
                 .items_center()
                 .justify_center()
-                .rounded_sm()
+                .rounded_xs()
                 .text_sm()
                 .text_color(fallback_color)
                 .border_1()