Update cupy to >= 7.8

[dantegd]

PR to update CuPy to version 7.8 to keep versions in sync (cuML requires 7.8 as of rapidsai/cuml#3378 due to usage of cupy.full order parameter) and corresponds to integration PR rapidsai/integration#208

cc @ajschmidt8

[ajschmidt8]

rerun tests

[codecov]

Codecov Report

Merging #7166 (9428f10) into branch-0.18 (8860baf) will increase coverage by 0.08%.
The diff coverage is 76.47%.

@@               Coverage Diff               @@
##           branch-0.18    #7166      +/-   ##
===============================================
+ Coverage        82.09%   82.17%   +0.08%     
===============================================
  Files               97       97              
  Lines            16474    16589     +115     
===============================================
+ Hits             13524    13632     +108     
- Misses            2950     2957       +7     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/lists.py	91.66% <ø> (-0.09%)	⬇️
python/cudf/cudf/core/dataframe.py	90.49% <ø> (-0.22%)	⬇️
python/cudf/cudf/io/orc.py	86.89% <55.55%> (-1.51%)	⬇️
python/cudf/cudf/core/dtypes.py	89.50% <66.66%> (-0.88%)	⬇️
python/cudf/cudf/io/csv.py	91.66% <77.77%> (-1.67%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.08% <100.00%> (-0.33%)	⬇️
python/cudf/cudf/core/reshape.py	91.19% <100.00%> (+0.16%)	⬆️
python/cudf/cudf/core/series.py	91.10% <100.00%> (-0.06%)	⬇️
python/cudf/cudf/utils/cudautils.py	48.38% <0.00%> (-0.17%)	⬇️
... and 15 more

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[kkraus14]

Why do we need to restrict it to >=7.8.0 when cuDF doesn't use any newer features of CuPy?