[Administration | Milestones] Impossible to change the owner of a milestone

Affected version : 6.0.0.RELEASE

Observed behaviour

When a milestone is 'Restricted', it is no longer possible to change its owner. The arrow appears next to the field, but clicking on it does not open the field for modification.

Steps to reproduce

• Having a 'Restricted' scope milestone (created by a 'Project Manager' user)
• Try to change the owner of the milestone

Expected result

It should be possible to select a different owner for the milestone. For example :

dev analysis:

• In the console navigator, we can see <url_squash>/backend/milestones/possible-owners is failed due to corrupted data.
• I think it is the Map<String, List<User>> that at some point does not work. This is not a good practice to directly use the entity User, I propose to send only the data we need.