rubicon adapter: downgrade video.api to optional (#5280)

the code for this was released a while back

dev-docs/bidders/rubicon.md

@@ -168,11 +168,11 @@ The following video parameters are supported here so publishers may fully declar
 | playerSize| required | width, height of the player in pixels | [640,360] - will be translated to w and h in bid request | array<integers> |
 | mimes | required | List of content MIME types supported by the player (see openRTB v2.5 for options) | ["video/mp4"]| array<string>|
 | protocols | required | Supported video bid response protocol values <br />1: VAST 1.0 <br />2: VAST 2.0 <br />3: VAST 3.0 <br />4: VAST 1.0 Wrapper <br />5: VAST 2.0 Wrapper <br />6: VAST 3.0 Wrapper <br />7: VAST 4.0 <br />8: VAST 4.0 Wrapper | [2,3,5,6] | array<integers>|
-| api | required | Supported API framework values: <br />1: VPAID 1.0 <br />2: VPAID 2.0 <br />3: MRAID-1 <br />4: ORMMA <br />5: MRAID-2 | [2] |  array<integers> |
 | linearity | required | OpenRTB2 linearity. 1: linear (in-stream ad), 2: non-linear (overlay ad) | 1 | integer |
 | maxduration | recommended | Maximum video ad duration in seconds. | 30 | integer |
 | minduration | recommended | Minimum video ad duration in seconds | 6 | integer |
 | playbackmethod | recommended | Playback methods that may be in use. Only one method is typically used in practice. (see openRTB v2.5 section 5.10 for options)| [2]| array<integers> |
+| api | optional | Supported API framework values: <br />1: VPAID 1.0 <br />2: VPAID 2.0 <br />3: MRAID-1 <br />4: ORMMA <br />5: MRAID-2 | [2] |  array<integers> |
 | skip | optional | Indicates if the player will allow the video to be skipped, where 0 = no, 1 = yes. | 1 | integer |
 | skipafter| optional | Number of seconds a video must play before skipping is enabled; only applicable if the ad is skippable. | 6 | integer|
 | minbitrate | optional | Minimum bit rate in Kbps. | 300 | integer |