Support floating labels on .form-control-plaintext (#32840)

* Support floating labels on `.form-control-plaintext`

* Update floating-labels.md

* Apply suggestions from code review

Co-authored-by: XhmikosR <xhmikosr@gmail.com>
Co-authored-by: Mark Otto <otto@github.com>

scss/forms/_floating-labels.scss

@@ -2,6 +2,7 @@
   position: relative;
 
   > .form-control,
+  > .form-control-plaintext,
   > .form-select {
     height: $form-floating-height;
     line-height: $form-floating-line-height;
@@ -19,8 +20,8 @@
     @include transition($form-floating-transition);
   }
 
-  // stylelint-disable no-duplicate-selectors
-  > .form-control {
+  > .form-control,
+  > .form-control-plaintext {
     padding: $form-floating-padding-y $form-floating-padding-x;
 
     &::placeholder {
@@ -46,6 +47,7 @@
 
   > .form-control:focus,
   > .form-control:not(:placeholder-shown),
+  > .form-control-plaintext,
   > .form-select {
     ~ label {
       opacity: $form-floating-label-opacity;
@@ -59,5 +61,10 @@
       transform: $form-floating-label-transform;
     }
   }
-  // stylelint-enable no-duplicate-selectors
+
+  > .form-control-plaintext {
+    ~ label {
+      border-width: $input-border-width 0; // Required to properly position label text - as explained above
+    }
+  }
 }