Replace append-bottom-20 class boards

This replaces the append-bottom-20 css class
inside the boards app

app/assets/javascripts/boards/components/board_configuration_options.vue

@@ -42,7 +42,7 @@ export default {
 </script>
 
 <template>
-  <div class="append-bottom-20">
+  <div class="gl-mb-5">
     <label class="label-bold gl-font-lg" for="board-new-name">
       {{ __('List options') }}
     </label>

app/assets/javascripts/boards/components/board_form.vue

@@ -195,7 +195,7 @@ export default {
     <template #body>
       <p v-if="isDeleteForm">{{ __('Are you sure you want to delete this board?') }}</p>
       <form v-else class="js-board-config-modal" @submit.prevent>
-        <div v-if="!readonly" class="append-bottom-20">
+        <div v-if="!readonly" class="gl-mb-5">
           <label class="label-bold gl-font-lg" for="board-new-name">{{ __('Title') }}</label>
           <input
             id="board-new-name"