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/tasks_ui/src/modal.rs

@@ -500,7 +500,7 @@ impl PickerDelegate for TasksModalDelegate {
                 .h_8()
                 .p_2()
                 .justify_between()
-                .rounded_b_md()
+                .rounded_b_sm()
                 .bg(cx.theme().colors().ghost_element_selected)
                 .border_t_1()
                 .border_color(cx.theme().colors().border_variant)