Add aria-modal attribute to Dialog (V2) element (#2851)

* Add `aria-modal` to `Dialog` (V2)

* Create .changeset/giant-games-leave.md

---------

Co-authored-by: Cole Bemis <[email protected]>

Author: iansan5653

.changeset/giant-games-leave.md

@@ -0,0 +1,5 @@
+---
+"@primer/react": patch
+---
+
+Add `aria-modal` attribute to `Dialog` (V2) element

src/Dialog/Dialog.tsx

@@ -302,6 +302,7 @@ const _Dialog = React.forwardRef<HTMLDivElement, React.PropsWithChildren<DialogP
             role={role}
             aria-labelledby={dialogLabelId}
             aria-describedby={dialogDescriptionId}
+            aria-modal
             sx={sx}
           >
             {header}