Fix error when variable is computed

$input-border-width is set to `var(--bs-border-width)` which cannot be directly multiplied

Author: jonny5

app/assets/stylesheets/simple_form-bootstrap/_form_floating_labels.scss

@@ -70,7 +70,7 @@ $sf-floating-input-padding-y: $input-padding-y * 2 !default;
       background-color: $input-bg;
       padding: 0 0 0 $sf-floating-input-padding-x;
       margin-top: $input-border-width;
-      margin-left: $input-border-width * 2;
+      margin-left: calc($input-border-width * 2);
       width: 99%;
     }
   }
@@ -99,4 +99,4 @@ $sf-floating-input-padding-y: $input-padding-y * 2 !default;
       color: #777;
     }
   }
-}
+}