Updated some of the golden image tests that were temporarily changed in https://github.com/flutter/flutter/pull/128914 to have M2 and M3 versions. 

Updated the linear_gradient_0 example to M3 (which will require updating its golden image as well).

More info in https://github.com/flutter/flutter/issues/127064

This reverts commit 5fd259be.

Adds Navigator.maybeOf to replace calling Navigator.of(context, nullOk: true), and removes the nullOk parameter. Also changes Navigator.of to return a non-nullable value, and removes many (120!) instances of the ! operator, reducing the possible places where a null dereference could occur.