Remove onUnhandledRequest middleware option

[baoshan]

This PR is based on the idea that Octokit middlewares (not limited to oauth-app.js) respect below proposed principle:

1. handle all requests matching pathPrefix (returns a predictable 404 response for unknown routes)
2. do not handle requests outside pathPrefix (but could be easily customized in an idiomatic way of the specific framework/runtime)

This PR also removes the (already deprecated) Cloudflare handler because this is a breaking change which leads to a major version bump.

[gr2m]

I suggest we release this as a beta to verify that @octokit/app still works without the onUnhandledRequest hook and to verify that you can easily build middleware.

[baoshan]

There is also a draft PR for app.js. I’d appreciate it if someone could give both a review.

[gr2m]

I've updated both PRs to merge into beta, so we are save to merge.

Can you add BREAKING CHANGE: ... sections to the pull request description so we don't miss that when we merge?

[baoshan]

Thanks.

I’ve appended a BREAKING CHANGE: line to the commit message if that’s what you mean.

[github-actions]

🎉 This PR is included in version 5.0.0-beta.1 🎉

The release is available on:

• GitHub release
• npm package (@beta dist-tag)

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