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/welcome/src/multibuffer_hint.rs

@@ -138,7 +138,7 @@ impl Render for MultibufferHint {
             .bg(cx.theme().status().info_background.opacity(0.5))
             .border_1()
             .border_color(cx.theme().colors().border_variant)
-            .rounded_md()
+            .rounded_sm()
             .overflow_hidden()
             .child(
                 h_flex()