Update checkbox display from block to flex

In V5 of the design system, checkboxes use flexbox, `display: flex`.

In the option select component, visibility of the checkbox is set using either `display: none` or `display: block`. Setting the value to `block` caused rendering issues when focusing a checkbox item. Setting the value to `flex` aligns with the approach used in the design system and fixes the rendering issue.

app/assets/javascripts/govuk_publishing_components/components/option-select.js

@@ -140,7 +140,7 @@ window.GOVUK.Modules = window.GOVUK.Modules || {};
     }
 
     for (i = 0; i < showCheckboxes.length; i++) {
-      obj.$allCheckboxes[showCheckboxes[i]].style.display = 'block'
+      obj.$allCheckboxes[showCheckboxes[i]].style.display = 'flex'
     }
 
     var lenChecked = obj.$optionsContainer.querySelectorAll('.govuk-checkboxes__input:checked').length