Migration guide for Dart/Flutter

[PeloWriter]

determine if we need a migration guide

ideally yes, migrating the Dart v3 to Dart v4 or Flutter v4, but we should not block this PR by that

Originally posted by @marandaneto in #3187 (comment)

[ueman]

Breaking Changes so far between Sentry v4 and (upcoming) v5:

• nullability related changes: Most static methods of Sentry require non-null values
• Classes were renamed: Fix: Rename some protocol classes sentry-dart#357

[ueman]

Migration Guide Draft: #3196