Fastify V4->V5 Upgrade

[ladderschool]

V4 of Fastify is approaching end of life by 6/30/25. This PR seeks to upgrade the relevant packages.
Fastify V5 also requires node>=20 now.

Migration docs:
https://fastify.dev/docs/latest/Guides/Migration-Guide-V5/

Luckily many changes were already deprecated from V3->V4 and I have found no issues running V5 in my production app. The only breaking change is how Fastify handles the logger, which this PR addresses, specifically in the createServerHelpers.ts file.

// v5
const loggerInstance = require('pino')();
const fastify = require('fastify')({
  loggerInstance
});

I have a test repository that I used to confirm these updates in this PR from:
https://github.com/ladderschool/fastify-v5-test

Local build succeeded: https://cloud.nx.app/runs/saVKz8Qhgn
Local tests succeeded: https://cloud.nx.app/runs/5SujFvDgYM

I did notice though that I had to change the imports to:

import Fastify from 'fastify/fastify'

Could be a module resolution issue in tsconfig that I have? The Fastify docs don't mention anything in regards to that, though.

[ladderschool]

I think @Tobbe's implementation in CedarJS is probably more type safe and easier to maintain down the line:

function isCustomLoggerInstance(
  logger: FastifyServerOptions['logger'] | FastifyServerOptions['loggerInstance'],
): logger is FastifyServerOptions['loggerInstance'] {
  return !!logger && typeof logger === 'object' && 'info' in logger;
}

const logger = options.logger ?? defaults.logger;

const resolvedOptions: ResolvedOptions = {
  apiRootPath: options.apiRootPath ?? defaults.apiRootPath,
  fastifyServerOptions: options.fastifyServerOptions ?? {
    requestTimeout: defaults.fastifyServerOptions.requestTimeout,
    logger: options.logger ?? defaults.logger,
    bodyLimit: defaults.fastifyServerOptions.bodyLimit,
  },
};

if (isCustomLoggerInstance(logger)) {
  resolvedOptions.fastifyServerOptions.loggerInstance = logger;
} else {
  resolvedOptions.fastifyServerOptions.logger = logger;
}

[Tobbe]

@ladderschool Do you want to update this PR to incorporate the improved implementation I have in CedarJS plus fixing the merge issues in yarn.lock, or are you happy to just use Cedar instead?