fix: Use identifier for export instead of field identifier

[iamareebjamal]

Fixes fossasia/open-event-frontend#5643

field_identifier is saved as jobTitle instead of job_title for absolutely no good reason (other than weird JS centric approach to everything in the stack?), hence any field like country and firstname exported correctly, but Job Title didn't. That's why I had created identifier property previously which changes camel case to snake case and matches the attributes on attendee. Using this instead of field_identifier fixes the issue