Replace _uid reference with uniqueId

We shouldn't access private props

ee/app/assets/javascripts/oncall_schedules/components/rotations/components/rotation_assignee.vue

 <script>
 import { GlAvatar, GlPopover } from '@gitlab/ui';
+import { uniqueId } from 'lodash';
 import { formatDate } from '~/lib/utils/datetime_utility';
 import { truncate } from '~/lib/utils/text_utility';
 import { __, sprintf } from '~/locale';
@@ -57,8 +58,7 @@ export default {
       });
     },
     rotationAssigneeUniqueID() {
-      const { _uid } = this;
-      return `${this.assignee.user.id}-${_uid}`;
+      return uniqueId('rotation-assignee-');
     },
     rotationMobileView() {
       return this.shiftWidth <= SHIFT_WIDTHS.xs;

ee/spec/frontend/oncall_schedule/rotations/components/rotation_assignee_spec.js

@@ -8,6 +8,8 @@ import { formatDate } from '~/lib/utils/datetime_utility';
 import { truncate } from '~/lib/utils/text_utility';
 import mockRotations from '../../mocks/mock_rotation.json';
 
+jest.mock('lodash/uniqueId', () => (prefix) => `${prefix}fakeUniqueId`);
+
 describe('RotationAssignee', () => {
   let wrapper;
 
@@ -71,9 +73,7 @@ describe('RotationAssignee', () => {
     });
 
     it('should render an assignee schedule and rotation information in a popover', () => {
-      // eslint-disable-next-line no-underscore-dangle
-      const UID = wrapper.vm._uid;
-      expect(findPopOver().attributes('target')).toBe(`${assignee.participant.user.id}-${UID}`);
+      expect(findPopOver().attributes('target')).toBe('rotation-assignee-fakeUniqueId');
       expect(findStartsAt().text()).toContain(formattedDate(assignee.startsAt));
       expect(findEndsAt().text()).toContain(formattedDate(assignee.endsAt));
     });

ee/spec/frontend/oncall_schedule/schedule/components/__snapshots__/rotations_list_section_spec.js.snap

@@ -83,7 +83,7 @@ exports[`RotationsListSectionComponent when the timeframe includes today renders
             <div
               class="gl-h-6 gl-bg-data-viz-blue-500 gl-display-flex gl-justify-content-center gl-align-items-center"
               data-testid="rotation-assignee"
-              id="1-12"
+              id="rotation-assignee-2"
             >
               <div
                 class="gl-text-white gl-display-flex gl-justify-content-center gl-align-items-center"
@@ -123,7 +123,7 @@ exports[`RotationsListSectionComponent when the timeframe includes today renders
             <div
               class="gl-h-6 gl-bg-data-viz-orange-500 gl-display-flex gl-justify-content-center gl-align-items-center"
               data-testid="rotation-assignee"
-              id="2-16"
+              id="rotation-assignee-3"
             >
               <div
                 class="gl-text-white gl-display-flex gl-justify-content-center gl-align-items-center"