Replace thrust::max_element with thrust::reduce in strings findall_re

[davidwendt]

This is a cleanup of findall.cu to change from using thrust::max_element to the more efficient thrust::reduce with a thrust::maximum operator. This also changes device_vector usage to device_uvector and adds more const to variable decls.

[codecov]

Codecov Report

Merging #7428 (0c91d44) into branch-0.19 (43b44e1) will increase coverage by 0.42%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.19    #7428      +/-   ##
===============================================
+ Coverage        81.80%   82.23%   +0.42%     
===============================================
  Files              101      101              
  Lines            16695    17060     +365     
===============================================
+ Hits             13658    14029     +371     
+ Misses            3037     3031       -6     

Impacted Files	Coverage Δ
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/comm/serialize.py	0.00% <0.00%> (ø)
python/cudf/cudf/_fuzz_testing/io.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/column/struct.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/_version.py	0.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_csv.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_orc.py	100.00% <0.00%> (ø)
python/dask_cudf/dask_cudf/io/tests/test_json.py	100.00% <0.00%> (ø)
...ython/dask_cudf/dask_cudf/io/tests/test_parquet.py	100.00% <0.00%> (ø)
python/cudf/cudf/core/join/join.py	95.93% <0.00%> (+0.02%)	⬆️
... and 37 more

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[codereport]

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[codereport]

@gpucibot merge