Sync plain text font-size with $input-font-size (#28464)

scss/_forms.scss

@@ -113,6 +113,7 @@ select.form-control {
   padding-top: $input-padding-y;
   padding-bottom: $input-padding-y;
   margin-bottom: 0; // match inputs if this class comes on inputs with default margins
+  @include font-size($input-font-size);
   line-height: $input-line-height;
   color: $input-plaintext-color;
   background-color: transparent;