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/src/window/prompts.rs

@@ -135,7 +135,7 @@ impl Render for FallbackPromptRenderer {
                     .border_1()
                     .border_color(opaque_grey(0.2, 0.5))
                     .mt_1()
-                    .rounded_sm()
+                    .rounded_xs()
                     .cursor_pointer()
                     .text_sm()
                     .child(action.clone())