/openrtb2/video endpoint deprecation

[Net-burst]

The legacy video endpoint has been largely unused and unmaintained for a long time, as OpenRTB now supports everything that is needed for video. As a result, this is now essentially dead code, which we should remove.

The proposal is to officially mark it as deprecated in PBS 4.0 and remove it in PBS 5.0.

[bsardo]

Backlog group agreed that we should update the endpoint to by default respond with a message indicating deprecated with information on how to reenable traditional responses. Traditional responses will be enabled via a host config

Host config:

• video.enable-deprecated-endpoint: false

Default response:

• status code: 501 (Not Implemented)
• message: "The video endpoint is deprecated and will be removed in 5.0. You may re-enable it via [the switch]"