Problem/Motivation
The composer file currently indicates that Drush releases older than 8.1.10 are not compatible with Drupal 9. However, this should be updated to indicate that Drush 9 and older are not compatible.
Proposed resolution
Update the "conflict" item in composer.json to list Drush 9 or older instead of 8.1.10.
Remaining tasks
Update composer.son.
User interface changes
n/a
API changes
n/a
Data model changes
n/a
Release notes snippet
tbd