Move "Repository Download Started" audit event to stream only

Description

"Repository Download Started" audit events are stored in the database and displayed at the group's audit events dashboard. These events are not too useful for auditing purpose and therefore they should be moved to stream only events.

Steps to reproduce

1. Ensure you have an ultimate license
2. Without logging in visit a public project and download the repository.
3. Login as the group owner and check the audit events dashboard for that group. It should contain audit events similar to the screenshot above.

Implementation Plan

1. Create a new event public_repository_download_operation and make it a streaming only event. This event will be logged every time a public project is downloaded.
2. Update the existing event repository_download_operation to only log events for non public projects (These could be private as well as internal projects).
3. Update the related RSpecs if required.