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ARROW-10661: [C#] Fix benchmarking project

After the upgrade to 3.1, the benchmarks stopped working since the version of BenchmarkDotNet used 2.1.
Also the writer benchmark reaches the 2gb limit of memory stream which causes an exception.

BenchmarkDotNet version is updated to 0.12.1
ColumnSetCount is reduced to 20, 21 was found out to be the maximum before reaching the 2gb.

@eerhardt sorry to bother again, but the last change broke the benchmark, didnt test it.. but here is the fix atleast.

Closes #8719 from Ulimo/ARROW-10661_fix_benchmark

Lead-authored-by: Ulimo <alexander.ostman@hotmail.com>
Co-authored-by: Östman Alexander <alexander.ostman@sweco.se>
Signed-off-by: Eric Erhardt <eric.erhardt@microsoft.com>