chore: Silence warning about missing moment.js library

What does this MR do?

We dropped moment.js library some time ago and pikaday library causes the following warning to appear in the build log:

WARNING in ./node_modules/pikaday/pikaday.js
Module not found: Error: Can't resolve 'moment' in '/builds/gitlab-org/gitlab-ui/node_modules/pikaday'
 @ ./node_modules/pikaday/pikaday.js
 @ ./node_modules/babel-loader/lib??ref--6!./node_modules/vue-loader/lib??vue-loader-options!./src/components/base/datepicker/datepicker.vue?vue&type=script&lang=js&
 @ ./src/components/base/datepicker/datepicker.vue?vue&type=script&lang=js&
 @ ./src/components/base/datepicker/datepicker.vue
 @ ./index.js
 @ ./src/components/base/form/form_input_group/form_input_group.stories.js
 @ ./src/components/base/form/form_input_group sync nonrecursive ^\.\/(?:(?:form_input_group\.stories\.js)$)$
 @ ./.storybook/generated-entry.js
 @ multi ./node_modules/@storybook/core/dist/server/common/polyfills.js ./node_modules/@storybook/core/dist/server/preview/globals.js ./.storybook/preview.js ./node_modules/@storybook/addon-knobs/dist/preset/addDecorator.js ./.storybook/generated-entry.js

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• Added the ~"component:*" label(s) if applicable.

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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