Move boards-sidebar class into content_sidebar

- The introduction of mounting-portal caused issue
  where this class was no longer getting added.
- No changelog needed since it is fixes in same release.

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

@@ -125,14 +125,9 @@ export default {
 
     <board-content-sidebar
       v-if="isSwimlanesOn || glFeatures.graphqlBoardLists"
-      class="boards-sidebar"
       data-testid="issue-boards-sidebar"
     />
 
-    <epic-board-content-sidebar
-      v-else-if="isEpicBoard"
-      class="boards-sidebar"
-      data-testid="epic-boards-sidebar"
-    />
+    <epic-board-content-sidebar v-else-if="isEpicBoard" data-testid="epic-boards-sidebar" />
   </div>
 </template>

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

@@ -79,7 +79,7 @@ export default {
       v-if="showSidebar"
       v-bind="$attrs"
       :open="isSidebarOpen"
-      class="gl-absolute"
+      class="boards-sidebar gl-absolute"
       @close="handleClose"
     >
       <template #header>{{ __('Issue details') }}</template>

ee/app/assets/javascripts/boards/components/epic_board_content_sidebar.vue

@@ -48,7 +48,7 @@ export default {
     <gl-drawer
       v-if="showSidebar"
       v-bind="$attrs"
-      class="gl-absolute"
+      class="boards-sidebar gl-absolute"
       :open="isSidebarOpen"
       @close="handleClose"
     >

ee/spec/frontend/boards/components/__snapshots__/board_content_sidebar_spec.js.snap

@@ -2,7 +2,7 @@
 
 exports[`ee/BoardContentSidebar matches the snapshot 1`] = `
 <div
-  class="gl-absolute"
+  class="boards-sidebar gl-absolute"
 >
   Issue details
   <boardsidebartitle-stub />