[ServiceBus] Update migration guide (Azure#15915)

* update migration guide

* Apply suggestions from code review

Co-authored-by: Rakshith Bhyravabhotla <rakshith.bhyravabhotla@gmail.com>
Co-authored-by: swathipil <76007337+swathipil@users.noreply.github.com>

* addressing comments

* tweak table of contents

* update according to comments

Co-authored-by: Rakshith Bhyravabhotla <rakshith.bhyravabhotla@gmail.com>
Co-authored-by: swathipil <76007337+swathipil@users.noreply.github.com>

Author: 3 people