`frontendApi` -> `publishableKey` as param to clerkPlugin

The frontendApi argument passed to clerkPlugin must be changed to publishableKey. Note that the values of the two keys are different, so both keys and values need to be changed. You can find your application's publishable key in the Clerk dashboard.

import { clerkPlugin } from '@clerk/fastify';

- fastify.register(clerkPlugin, { frontendApi: '...' });
+ fastify.register(clerkPlugin, { publishableKey: '...' });