update dropdown active states for bootstrap 4

app/assets/stylesheets/framework/gitlab_theme.scss

@@ -35,7 +35,7 @@
         }
         &.active > a,
-        &.dropdown.open > a {
+        &.dropdown.show > a {
           color: $color-900;
           background-color: $color-alternate;
         }
@@ -74,7 +74,7 @@
         }
         &.active > a,
-        &.dropdown.open > a {
+        &.dropdown.show > a {
           color: $color-900;
           background-color: $color-alternate;