[MRG] update the migration guide for 4.0 with version pinning instructions

[ctb]

Per #1016 (comment)

Note, this is a PR into #1283

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[luizirber]

I just saw this "upgrade process" in https://www.oxinabox.net/2021/02/13/Julia-1.6-what-has-changed-since-1.0.html#deprecations-are-muted-by-default and it might be useful to have something similar in the upgrade notes:

Solving deprecations is a thing you should actively choose to do rather than casually when trying to do something else. In particular, when updating to support a new major release of one of your dependencies, you should follow a process. Something like:

• Check the release notes.
• Relax compat bounds
• Run your integration tests, if everything passes you are done.
• If tests failed, revert the change to compat bounds, then rerun your integration tests paying attention to deprecation warnings.

Updating your dependencies should be an active choice. Potentially one that is automated, but not one that you do while adding another feature (if you can help it).

[ctb]

done! I restructured things a bit and removed some redundancy, too.