[A11y] Improve color contrast of focus outline on Firefox (#8295)

Improve the color contrast of Firefox's outline for focused items. This effectively reverts c245f2c as Firefox now properly outlines elements.

Chromium browsers (Chrome, Edgium, Opera) are all unaffected by this change. I also verified that Internet Explorer's focus outlines pass the 3:1 color contrast requirement.

Author: loic-sharma

src/Bootstrap/dist/css/bootstrap.css

@@ -4,8 +4,6 @@
   // WebKit-specific. Other browsers will keep their default outline style.
   // (Initially tried to also force default via `outline: initial`,
   // but that seems to erroneously remove the outline in Firefox altogether.)
-  // The next line came from: https://stackoverflow.com/a/38571103
-  outline: 2px auto Highlight;
   outline: 5px auto -webkit-focus-ring-color;
   outline-offset: -2px;
 }