Do not allow logger and loggerInstance

[NullVoxPopuli]

In this PR:

• Bug fix (non-breaking change which fixes an issue)

Issues reference:

I haven't reported an issue yet, because I just found out about the issue, and figured it'd be better to just fix it than to report an issue first.

You cannot provide both logger and loggerInstance. Please provide only one.'

this happens when passing:

const fileLogger = pino(pino.destination(turboLogsPath));

const fastifyApp = createApp({
  logger: fileLogger,
});

Noticed with an accidental upgrade to 2.2.8 (from 2.2.1), which includes the loggerInstance change from 2.2.7, here:

• FastifyError [Error]: logger options only accepts a configuration object. #479

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