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/diagnostics/src/diagnostics.rs

@@ -973,7 +973,7 @@ fn diagnostic_header_renderer(diagnostic: Diagnostic) -> RenderBlock {
                 h_flex()
                     .gap_2()
                     .px_1()
-                    .rounded_md()
+                    .rounded_sm()
                     .bg(color.surface_background.opacity(0.5))
                     .map(|stack| {
                         stack.child(