Make the BETA CUDA interface the default

[NicolasHug]

We announced in 0.11 that the BETA CUDA interface will become the default. Let's do it.

In the following I'm going to refer to the 'Beta' interface as the 'default' interface. The 'ffmpeg' interface is still the 'ffmpeg' interface.

In terms of UX, I think we want:

dec = VideoDecoder(..., device="cuda")  # now relies on the 'default' interface (i.e. 'beta'). Previously this relied on 'ffmpeg'

# to manually select it:
with set_cuda_backend("default"): 
    VideoDecoder(...)

# still works for BC, but we should stop document this as officially supported:
with set_cuda_backend("beta"): 
    VideoDecoder(...)


# still works. This is now the only way to get the ffmpeg interface.
with set_cuda_backend("ffmpeg"):  
    VideoDecoder(...)

We'll have to do some small plumbing changes too, particularly to the tests. And we should also update our docs to stop mentioning "beta". This can be done in a few different PRs.

Eventually, we might want to deprecate and stop exposing the 'ffmpeg' interface publicly, and only have it for tests, but that's a discussion that's out of scope for this issue.