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/git_ui/src/askpass_modal.rs

@@ -74,7 +74,7 @@ impl Render for AskPassModal {
                     .px(DynamicSpacing::Base12.rems(cx))
                     .pt(DynamicSpacing::Base08.rems(cx))
                     .pb(DynamicSpacing::Base04.rems(cx))
-                    .rounded_t_md()
+                    .rounded_t_sm()
                     .w_full()
                     .gap_1p5()
                     .child(Icon::new(IconName::GitBranch).size(IconSize::XSmall))