Replace `.is` method in `spec/frontend/work_items/components/work_item_notes_spec.js`

In VTU 1, is method to check on component instance or HTML tag is deprecated.

If you are using shallowMount, you can replace is with element.tagName:

-        expect(asset.is(GlLink)).toBe(true);
+        expect(asset.element.tagName).toBe('GL-LINK');

If you are using mount and you had is with component instance, you may want to use component name instead:

-        expect(asset.is(GlLink)).toBe(true);
+        expect(asset.vm).toBeTruthy();
+        expect(asset.vm.$options.name).toBe('GlLink');