Switch to React's isValidElement

[Tobbe]

@jtoar pointed out in #3977 that React has isValidElement that we can use instead of our own implementation. #3977 got closed, but I didn't want to forget about isValidElement, so this PR introduces just that one change

[replay-io]

16 replays were recorded for 6d9b265.

0 Failed

16 Passed

requireAuth graphql checks

locator.waitFor: Target closed
=========================== logs ===========================
waiting for locator('.rw-form-error-title').locator('text=You don\'t have permission to do that') to be visible
============================================================

• useAuth hook, auth redirects checks
• Check that tags are rendering the correct dynamic data
• Check that a specific blog post is prerendered
• Check that about is prerendered
• Check that homepage is prerendered
• Check that you can navigate from home page to specific blog post
• Waterfall prerendering (nested cells)
• RBAC: Admin user should be able to delete contacts
• RBAC: Should not be able to delete contact as non-admin user
• Smoke test with dev server
• Smoke test with rw serve
• Loads Cell mocks when Cell is nested in another story
• Loads Cell Stories
• Loads MDX Stories
• Mocks current user, and updates UI while dev server is running

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