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/keybinding.rs

@@ -120,7 +120,7 @@ impl RenderOnce for KeyBinding {
                 h_flex()
                     .flex_none()
                     .py_0p5()
-                    .rounded_sm()
+                    .rounded_xs()
                     .text_color(cx.theme().colors().text_muted)
                     .when(use_text, |el| {
                         el.child(