[libc++] Mark <stdatomic.h> as requiring C++23

Otherwise, we might get errors with modules in pre-C++23 when mixing
<atomic> and <stdatomic.h>. This should fix breakage on Green Dragon.

NOKEYCHECK=True
GitOrigin-RevId: 4128a4a6c219a6c83483d4538a59ed8f523868a5
diff --git a/include/module.modulemap b/include/module.modulemap
index 6cf0467..7ae3d10 100644
--- a/include/module.modulemap
+++ b/include/module.modulemap
@@ -51,6 +51,7 @@
       export *
     }
     module stdatomic_h {
+      requires cplusplus23
       header "stdatomic.h"
       export *
     }