Pull latest Kyber version from upstream

[bhess]

Pull latest Kyber version from upstream, which fixes a potential constant-time issue in the "ref" version as reported here:
https://groups.google.com/a/list.nist.gov/d/msgid/pqc-forum/ZXxUpck1501jEcRq%40localhost

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider and OQS-OpenSSH will also need to be ready for review and merge by the time this is merged.)

[baentsch]

fixes a potential constant-time issue in the "ref" version

I assume it's intentional that a fix to "ref" code also changes "avx2" files?!

[bhess]

I assume it's intentional that a fix to "ref" code also changes "avx2" files?!

Good point. The update includes two separate commits upstream. The avx2 change is from the earlier commit: pq-crystals/kyber@9c32260.