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/copilot/src/sign_in.rs

@@ -122,7 +122,7 @@ impl CopilotCodeVerification {
             .p_1()
             .border_1()
             .border_muted(cx)
-            .rounded_md()
+            .rounded_sm()
             .cursor_pointer()
             .justify_between()
             .on_mouse_down(gpui::MouseButton::Left, {