[3.3] Update "Getters can’t be promoted" for status in Dart 3.3

[parlough]

Section to update: https://dart.dev/tools/non-promotion-reasons#not-field

The discussion and following note will need to be updated to account for the expansion of type promotion to some abstract getters (dart-lang/sdk#54056).

Since the destination will still be linked to by previous releases of Dart, make sure to leave a version note for before 3.3. Another option is to introduce a new section and link for 3.3 and maintain the existing section for earlier versions.

[parlough]

Fixed by 59b4d26. Thanks Marya!

The commit didn't add a mention of which version this changed in, but I think it's probably fine in this case as the note is an exception to the above primary documentation. If anyone raises it as a point of confusion or feels strongly, we can adjust it further :)