Deprecate GlSkeletonLoading component

Will be replaced with GlSkeletonLoader, but for not this just
indicates that GlSkeletonLoading is deprecated

spec/frontend/clusters_list/components/clusters_spec.js

 import MockAdapter from 'axios-mock-adapter';
 import { mount } from '@vue/test-utils';
-import { GlLoadingIcon, GlPagination, GlSkeletonLoading, GlTable } from '@gitlab/ui';
+import {
+  GlLoadingIcon,
+  GlPagination,
+  GlDeprecatedSkeletonLoading as GlSkeletonLoading,
+  GlTable,
+} from '@gitlab/ui';
 import * as Sentry from '@sentry/browser';
 import axios from '~/lib/utils/axios_utils';
 import Clusters from '~/clusters_list/components/clusters.vue';

spec/frontend/issuables_list/components/issuables_list_app_spec.js

 import axios from 'axios';
 import MockAdapter from 'axios-mock-adapter';
 import { shallowMount } from '@vue/test-utils';
-import { GlEmptyState, GlPagination, GlSkeletonLoading } from '@gitlab/ui';
+import {
+  GlEmptyState,
+  GlPagination,
+  GlDeprecatedSkeletonLoading as GlSkeletonLoading,
+} from '@gitlab/ui';
 import waitForPromises from 'helpers/wait_for_promises';
 import { TEST_HOST } from 'helpers/test_constants';
 import { deprecatedCreateFlash as flash } from '~/flash';

spec/frontend/releases/components/app_show_spec.js

 import Vuex from 'vuex';
 import { shallowMount } from '@vue/test-utils';
-import { GlSkeletonLoading } from '@gitlab/ui';
+import { GlDeprecatedSkeletonLoading as GlSkeletonLoading } from '@gitlab/ui';
 import ReleaseShowApp from '~/releases/components/app_show.vue';
 import { release as originalRelease } from '../mock_data';
 import ReleaseBlock from '~/releases/components/release_block.vue';

spec/frontend/repository/components/table/index_spec.js

 import { shallowMount } from '@vue/test-utils';
-import { GlSkeletonLoading, GlButton } from '@gitlab/ui';
+import { GlDeprecatedSkeletonLoading as GlSkeletonLoading, GlButton } from '@gitlab/ui';
 import Table from '~/repository/components/table/index.vue';
 import TableRow from '~/repository/components/table/row.vue';

spec/frontend/vue_mr_widget/components/terraform/mr_widget_terraform_container_spec.js

-import { GlSkeletonLoading, GlSprintf } from '@gitlab/ui';
+import { GlDeprecatedSkeletonLoading as GlSkeletonLoading, GlSprintf } from '@gitlab/ui';
 import { shallowMount } from '@vue/test-utils';
 import MockAdapter from 'axios-mock-adapter';
 import { invalidPlanWithName, plans, validPlanWithName } from './mock_data';

spec/frontend/vue_shared/components/user_popover/user_popover_spec.js

-import { GlSkeletonLoading, GlSprintf, GlIcon } from '@gitlab/ui';
+import { GlDeprecatedSkeletonLoading as GlSkeletonLoading, GlSprintf, GlIcon } from '@gitlab/ui';
 import { shallowMount } from '@vue/test-utils';
 import UserPopover from '~/vue_shared/components/user_popover/user_popover.vue';