Improve migration guide re v20

[mroderick]

In v20, we removed fakeXHR and fakeServer.

It's time. We might want to add some notes to the migration-guide.md about alternatives (msw, etc).

Originally posted by @fatso83 in https://github.com/sinonjs/sinon/pull/2642#pullrequestreview-2698737989

[humitos]

Today I found that my tests don't run anymore after upgrading to 20.x because of fake server not being available. I'm don't have too much experience on JavaScript, so I'd appreciate any guidance on what are good alternatives here so I can re-write my tests.

[fatso83]

@humitos the library (nise) is still available. Working just as well. Just not bundled. Just import it directly. Same API.

[humitos]

@fatso83 thanks a lot for your comment. I was able to use nise with very few modifications (just variable names changes) 👍🏼

[mohamedmansour]

Is anyone going to update the migration guide docs for v20, cause it is still v19, otherwise I can help docs/guides/migration-guide.md

[fatso83]

@mohamedmansour Please do! I do not have much time (or incentive) to maintain and work on JS related projects at the moment, as I have almost exclusively been working with Ada, Kotlin/JVM and Python in the last year.