Update CSV attachments import documentation

With the move to Vertigo, there is no longer a file system to upload attachments to for CSV imports which renders the following documentation obsolete:

 https://confluence.atlassian.com/jirakb/importing-attachments-into-jira-cloud-applications-using-csv-fails-779160784.html

Instead, users need to host the attachments on a publicly accessible server and update the CSV file to link to the attachments as shown on the following documentation instead:

https://confluence.atlassian.com/adminjiracloud/importing-data-from-csv-776636762.html