Update 5.0 version in Migration Guide

[crandmck]

Docs in en and de (in https://github.com/expressjs/expressjs.com/pull/1371) were updated, but all other langs need update as well to change:

$ npm install [email protected] --save

to

$ npm install "express@>=5.0.0-beta.1" --save

Perhaps we should consider a better way to address this in the long run that wouldn't require touching so many files. The _data/express.yml file has a current_version version for the latest 4.x release version. We could add next_version for the 5.x latest version. Of course, when 5.x is released, current_version would become 5.0.0 or whatever, and next_version would be something like 6.0.0-alpha.1 or the like.