gpui: Rename rounded_md to rounded_sm (#26228)

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

Follow up to https://github.com/zed-industries/zed/pull/26221, which
freed up the `rounded_sm` name.

Release Notes:

- N/A

crates/component_preview/src/component_preview.rs

@@ -108,7 +108,7 @@ impl ComponentPreview {
                 v_flex()
                     .border_1()
                     .border_color(cx.theme().colors().border)
-                    .rounded_md()
+                    .rounded_sm()
                     .w_full()
                     .gap_4()
                     .py_4()