Add targeting .Net 8.0 for DataFrame package

[asmirnov82]

Fixes #7167

[JakeRadMSFT]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[asmirnov82]

TestTokenizerUsingExternalVocab test fails, because external vocabulary is not available by https://pythia.blob.core.windows.net/public/encoding/gpt2.tiktoken url

[JakeRadMSFT]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 68.67%. Comparing base (0c51cc6) to head (016dda3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7168      +/-   ##
==========================================
- Coverage   68.94%   68.67%   -0.27%     
==========================================
  Files        1397     1263     -134     
  Lines      266970   257955    -9015     
  Branches    27600    26698     -902     
==========================================
- Hits       184057   177146    -6911     
+ Misses      75807    73996    -1811     
+ Partials     7106     6813     -293     

Flag	Coverage Δ
Debug	68.67% <25.00%> (-0.27%)	⬇️
production	62.94% <ø> (-0.59%)	⬇️
test	88.88% <25.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...osoft.Data.Analysis.Tests/ArrowIntegrationTests.cs	100.00% <100.00%> (ø)
...est/Microsoft.Data.Analysis.Tests/ArrayComparer.cs	63.55% <0.00%> (ø)

... and 139 files with indirect coverage changes