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/git_ui/src/git_panel.rs

@@ -3216,7 +3216,7 @@ impl SplitButton {
 impl RenderOnce for SplitButton {
     fn render(self, _window: &mut Window, cx: &mut App) -> impl IntoElement {
         h_flex()
-            .rounded_md()
+            .rounded_sm()
             .border_1()
             .border_color(cx.theme().colors().text_muted.alpha(0.12))
             .child(self.left)