Adjust sync_seat_link action to respond with JSON

What does this MR do?

Related to #325156

This MR adjusts the Admin::LicensesController#sync_seat_link controller action to respond with a JSON instead of a redirect. This allows the FE to leverage async actions instead of a full page refresh.

After discussion with FE, we decided to remove the messages in the response from the BE. This will be handled in the FE.

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