Update the Attachment Storage Configuration documentation about attachment migration

According to the documentation: https://confluence.atlassian.com/doc/attachment-storage-configuration-166876.html ,

Database Attachment Storage has been deprecated.

However, there's still an instruction to migrate to Database Attachment Storage:

Migrating between attachment storage systems

You can migrate your attachments from one storage system to another. All existing attachments will be moved over to the new attachment storage system.

This could be quite misleading to customer, as they will think that "even though it's deprecated". they're still be able to migrate.

However the comment on this bug report say otherwise: https://jira.atlassian.com/browse/CONF-33691