Make username for events consistent

[peetucket]

We use user_name to indicate the user that is taking an action for recording in events in some cases (see #5366), but we use opening_user_name for the open event. We should consider making this user_name throughout.

Will require

• changing DSA for the open action
• changing dsa-client for the open call
• looking up any consumers of the dsa-client open endpoint and updating them

see https://github.com/search?q=repo%3Asul-dlss%2Fdor-services-app%20opening_user_name&type=code and https://github.com/search?q=repo%3Asul-dlss%2Fdor-services-app%20opening_user_name&type=code and overall: https://github.com/search?q=org%3Asul-dlss%20opening_user_name&type=code