xwing: fix mlkem768 keys

[stsch9]

trafficstars

Hi,

since some changes were made in the mlkem768 package, I tried to fix the xwing package. I hope this is helpful for you.

[jmcrawford45]

@FiloSottile are there any plans to merge this in? I see there's been some activity in crypto/internal for the port of this package, so I'm not sure if this project should by considered abandoned or not at this point.

[boppreh]

Now that ML-KEM is standardized, this would be even more useful.

[FiloSottile]

Implemented draft-connolly-cfrg-xwing-kem-04 and matched the latest mlkem768 cached API.

/cc @dconnolly @bwesterb