Update cupy to >= 7.8

[dantegd]

Integration PR corresponding to NVIDIA/cuml#3378

[ajschmidt8]

reposting this comment from NVIDIA/cuml#3378 for awareness:

Also, looks like the spec will need to be adjusted in the other repos:

• https://github.com/rapidsai/cuxfilter/blob/branch-0.18/conda/recipes/cuxfilter/meta.yaml#L33
• https://github.com/rapidsai/cusignal/blob/branch-0.18/conda/recipes/cusignal/meta.yaml#L31
• https://github.com/rapidsai/cudf/blob/branch-0.18/conda/recipes/cudf/meta.yaml#L38

I see there are two open PRs currently:

• Update cupy to >= 7.8 cuxfilter#225
• Update cupy to >= 7.8 NVIDIA/cudf#7166

I don't see a cusignal PR yet, however it looks like @awthomp is also adjusting the cupy version spec in rapidsai/cusignal#292.

[dantegd]

@ajschmidt8 cusignal minimum version is already 8.0, so I didn't open a PR there due to that