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/extensions_ui/src/components/extension_card.rs

@@ -40,7 +40,7 @@ impl RenderOnce for ExtensionCard {
                 .bg(cx.theme().colors().elevated_surface_background)
                 .border_1()
                 .border_color(cx.theme().colors().border)
-                .rounded_md()
+                .rounded_sm()
                 .children(self.children)
                 .when(self.overridden_by_dev_extension, |card| {
                     card.child(