Ignore changes to required/optional status of assignee field in field configurations

Jira UI does not allow changing in a field configuration if the "assignee" field is requirable or mandatory. Instead, this is determined by the global setting to allow unassigned issues. If these are allowed, then the field is optional, otherwise it will be required. Note that, if unassigned issues are allowed, it is not possible to set the field to be required in some projects.

In order to have the import replicate this behaviour of Jira, it will just ignore any request to change the required/optional status of field assignee in field configurations. The previous implementation only produces a number of not very useful errors and warnings when the new settings for that field do not coincide with the existing "unassigned issues allowed" global setting.