Add possibility to define custom border color for disabled form controls (#29257)

scss/forms/_form-control.scss

@@ -59,6 +59,7 @@
   &:disabled,
   &[readonly] {
     background-color: $input-disabled-bg;
+    border-color: $input-disabled-border-color;
     // iOS fix for unreadable disabled content; see https://github.com/twbs/bootstrap/issues/11655.
     opacity: 1;
   }