fix(button): better accessibility for flat buttons in high-contrast (#9783)

* Improves the accessibility for `mat-flat-button` elements in high-contrast mode. Currently it's nearly impossible to figure out, whether it's a button or not.

Author: devversion

src/lib/button/button.scss

@@ -111,9 +111,10 @@
   z-index: 1;
 }
 
-// Add an outline to make it more visible in high contrast mode.
+// Add an outline to make buttons more visible in high contrast mode. Stroked buttons
+// don't need a special look in high-contrast mode, because those already have an outline.
 @include cdk-high-contrast {
-  .mat-button, .mat-raised-button, .mat-icon-button, .mat-fab, .mat-mini-fab {
+  .mat-button, .mat-flat-button, .mat-raised-button, .mat-icon-button, .mat-fab, .mat-mini-fab {
     outline: solid 1px;
   }
 }