auth-twitch: Remove API v5 reference in favour of Helix

[Marenthyu]

Intention

If my search of the repository is correct, the following line is the single (remaining) reference to Twitch's v5/kraken API, which is discouraged to be used and will be discontinued at some point soon™.

https://github.com/obsproject/obs-studio/blob/cab32e7e67c2a78d10e0961f746ee7408978fb43/UI/auth-twitch.cpp#L79

This should be replaced with authenticated calls to https://dev.twitch.tv/docs/api/reference#get-stream-key and https://dev.twitch.tv/docs/api/reference#get-users to get the stream key and user name, respectively.

Reason

This change ensures longer-term usability of this version of OBS, in anticipation of the v5 shutdown, which has not yet been announced but is still scheduled to happen eventually (aka SOON™); The addition of the v5 Migration Guide to the Twitch Dev Docs shows that it is still very much on the radar for Twitch.

Considerations

Notable changes for this:

• All calls to Helix require the Client-ID and Authorization headers with a valid OAuth token
  • Existing tokens will not need to be recreated, as mentioned here, so the existing authentication can be reused.
• The Accept header with the application/vnd.twitchtv.v5 value is no longer required and can be replaced with just application/json
• It is two distinct calls for this information as the stream key endpoint does not return the user name

[Marenthyu]

Fixed with 92f5afae5191ac97f8449b8f7ac57007ccb5b373