Hm, if the queue has EXECUTE AS OWNER, I think you should not change the queue activation, but the owner of the queue should be changed. Which could be the database owner. If you run
SELECT user_name(principal_id), * FROM sys.service_queues WHERE name = 'IdentityServerNotificationCleanup'
What does the first column say? What does the principal_id column say? If we have NULL, it is the database owner that should be changed:
ALTER AUTHORIZATION ON DATABASE::yourdb TO betterowner
If the query returns a non-NULL value, don't touch anything yet, since there are probably more objects you need to change ownership on.