add migration for pytorch 2.6

[h-vetinari]

After conda-forge/pytorch-cpu-feedstock#326. As I mentioned there, the plan is to roll out 2.6 across existing feedstocks first, and then do it again more slowly with a piggyback migrator that removes skip: true [win] (though of course feedstock maintainers are encouraged to try that out individually as well).

It's possible we still have some bugs to iron out, in particular:

• CMake metadata for CUDA builds not yet self-contained pytorch-cpu-feedstock#357
• Pytorch missing DLLs when installed into an environment on Windows, but not on linux pytorch-cpu-feedstock#354 (which needs switching our MKL to llvm-openmp, which needs unblocking MKL 2025).

Please raise issues on the pytorch feedstock if something fishy is going on in one of the upgrade PRs.

[conda-forge-admin]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe/meta.yaml) and found it was in an excellent condition.