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/ui/src/components/numeric_stepper.rs

@@ -75,7 +75,7 @@ impl RenderOnce for NumericStepper {
                 h_flex()
                     .gap_1()
                     .px_1()
-                    .rounded_sm()
+                    .rounded_xs()
                     .bg(cx.theme().colors().editor_background)
                     .child(
                         IconButton::new("decrement", IconName::Dash)