fix(material/core): unable to distinguish disabled mat-option in high contrast mode (#21484)

Fixes that disabled `mat-option` instances cannot be distinguished from enabled ones in
high contrast mode.

(cherry picked from commit e608da3)

Author: crisbeto

src/material/core/option/option.scss

@@ -44,6 +44,13 @@
       border: solid $high-contrast-border-width currentColor;
       margin: 0;
     }
+
+    // Fade out the option when it is disabled so that it can be distinguished from the enabled
+    // options. Note that ideally we'd use `color: GreyText` here which is what the browser uses
+    // for disabled buttons, but we can't because Firefox doesn't recognize it.
+    &[aria-disabled='true'] {
+      opacity: 0.5;
+    }
   }
 }