Improve .NET banner accessibility (#8978)

Our black focus outline does not work well on the "20 years of .NET" banner. This moves to a white dashed outline to align with develper.microsoft.com. I verified this passes Accessibility Insights FastPass (the previous focus outline also passed 🙁)

Part of https://github.com/NuGet/Engineering/issues/4226

Author: loic-sharma

src/Bootstrap/dist/css/bootstrap-theme.css

@@ -63,6 +63,13 @@
             color: white;
             font-weight: 600;
           }
+
+          a:focus {
+            outline-color: #fff;
+            outline-style: dashed;
+            outline-width: 2px;
+            outline-offset: 3px;
+          }
         }
       }
     }