Bump bootstrap to 4.3.1

style/bootstrap/mixins/_size.scss

@@ -1,6 +1,7 @@
-// Sizing shortcuts
-
-@mixin size($width, $height: $width) {
-  width: $width;
-  height: $height;
-}
+// Sizing shortcuts
+
+@mixin size($width, $height: $width) {
+  width: $width;
+  height: $height;
+  @include deprecate("`size()`", "v4.3.0", "v5");
+}