Deprecate @keystone-6/system and server.healthCheck

[dcousens]

This pull request deprecates @keystone-6/system, including each of the exports createSystem, createExpressServer and initConfig. Neither of these exports should be used by developers in new code, as we are slowly changing the way users interact with Keystone generally. If you have a usecase for these functions, please comment on this pull request!

This pull request additionally deprecates config.server.healthCheck, with the new recommendation for developers to use extendExpressApp with a custom route that fits their use-case. This configuration was highly opinionated and often unhelpful in different environments when you needed a very specific response format.
An implementation that wants to upgrade early could use the following:

import { config } from '@keystone-6/core';
import { TypeInfo } from '.keystone/types';

export default config<TypeInfo>({
  ...
  server: {
    extendExpressApp: (app, commonContext) => {
      app.get('/status', async (req, res) => {
        res.status(200).json({ status: 'up' })
      })
    },
  },
})

[borisno2]

Looks good!

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