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/ui/src/components/content_group.rs

@@ -82,7 +82,7 @@ impl RenderOnce for ContentGroup {
             .when(self.border, |this| {
                 this.border_1().border_color(cx.theme().colors().border)
             })
-            .rounded_md()
+            .rounded_sm()
             .p_2()
             .children(self.children)
     }