Fixing broken rake spotlight:export[exhibit_slug] task.

[phil-plencner-hl]

The Spotlight rake task for exporting exhibits is currently broken. When you run it, the result is similar to this:

$ bundle exec rake spotlight:export[my-exhibit] > export.json
rake aborted!
NameError: uninitialized constant Spotlight::ExhibitExportSerializer

It looks like back in April 2020 ExhibitExportSerializer was replaced with ExhibitImportExportService and the rake task was not updated to use the new service:

https://github.com/projectblacklight/spotlight/commit/1c61dff3d4dfe6694ac8a26bd3ade11784aae717

This PR updates spotlight_tasks.rake to use the new service.