migration: start of an editing pass (#6687)

* migration: start of an editing pass

* More progress on an editing pass

* make it build

* Reorganize and rework migration some more

* Apply suggestions from code review

Co-authored-by: Rose M Koron <[email protected]>

* migration: result of auditing the change in exported API

* migration: Add description of changes from #6355

* Reorganize error handling changes

* migration: gql tag

* Apply suggestions from code review

Co-authored-by: Rose M Koron <[email protected]>

* Apply suggestions from code review

Co-authored-by: Rose M Koron <[email protected]>

* Tweak gql section

including re-adding triple-quotes because GH suggestions don't like
them. And I guess trimming trailing newlines which I guess my editor
does and which got introduced recently?

* Simplify some bullets and add body-parser/cors section back

* A few @trevor-scheer related changes

* Apply suggestions from code review

Co-authored-by: Rose M Koron <[email protected]>

* Follow-up to the previous one

* gateway

* as shown above

* context

Co-authored-by: Rose M Koron <[email protected]>
Co-authored-by: Ivan Goncharov <[email protected]>