`frontendApi` -> `publishableKey` as prop to `ClerkProvider`

The frontendApi prop passed to ClerkProvider must be changed to publishableKey. The value also must be updated to be a publishable key rather than a frontend API key. Publishable keys can be found in your Clerk dashboard.

import { ClerkProvider } from '@clerk/clerk-expo';

- <ClerkProvider frontendApi='...' />
+ <ClerkProvider publishableKey='...' />