gpui: Rename rounded_sm to rounded_xs (#26221)

This PR renames the `rounded_sm` style method to `rounded_xs`.

This will allow us to add an additional step in the scale.

Release Notes:

- N/A

crates/markdown_preview/src/markdown_preview_view.rs

@@ -229,7 +229,7 @@ impl MarkdownPreviewView {
                                                 s.bg(cx.theme().colors().border_variant)
                                             }
                                         })
-                                        .rounded_sm();
+                                        .rounded_xs();
 
                                     container.child(
                                         div()