Fix readonly-plain-text with button addon (#25871)

Fixes #25870

Author: caiquecastro

scss/_input-group.scss

@@ -12,6 +12,7 @@
   width: 100%;
 
   > .form-control,
+  > .form-control-plaintext,
   > .custom-select,
   > .custom-file {
     position: relative; // For focus state's z-index