docs(guides/migration-guide-v4): update content

[Fdawgs]

This changes the following:

• General tidy and restructure of sentences to improve clarity; removes waffle
• Adds section regarding deprecation of variadic fastify.listen() method under the non-breaking changes header
• Renames "Deprecation of app.use()" section to "Removed app.use()" to reflect that the function has been fully removed and not just deprecated
• Updated example handlers to be inline with style used in "Getting-Started" guide

What the guide is still missing:

• An example under "Need to return reply to signal a "fork" of the promise chain" section
• Error handling changes, as noted in http error does not return the proper status code #3959

Checklist

• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[github-actions]

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