fix(material/stepper): add HCM disabled styles

This adds disabled styles for stepper header in high contrast mode

Author: jelbourn

src/material/stepper/step-header.scss

@@ -25,6 +25,17 @@
         text-decoration: underline;
       }
     }
+
+    // When a step header is disabled in high contrast mode, set the text color to the disabled
+    // color, which is (unintuitively) named "GrayText".
+    &[aria-disabled="true"] {
+      outline-color: GrayText;
+      .mat-step-label,
+      .mat-step-icon,
+      .mat-step-optional {
+        color: GrayText;
+      }
+    }
   }
 }