Update Projects to .NET 8 in MLNET 4.0 Branch

[JakeRadMSFT]

• This PR drops netfx and .netstandard support.
• ML.NET 4.0 will target .NET 8
• Fixes various issues hit targeting .NET 8
• Cleans up build/project files to remove multi-targeting related things (probably still more work to do here)

TODO:

• [ ] Open bug for looking at our code analysis things
• [ ] Open bug enabling the nullable things for entire repo
• [ ] Open bug to consolidate to a shared ApiCompatSuppressionFile
  • Reference: https://github.com/search?q=repo%3Adotnet%2Fruntime%20ApiCompatSuppressionFile&type=code
• [ ] Open bug for removing TargetFramework/NET8 from Build Definition

[codecov[bot]]

Codecov Report

Merging #6749 (1c49d1e) into feature/4.0 (b48e9b7) will increase coverage by 0.23%. The diff coverage is 76.19%.

@@               Coverage Diff               @@
##           feature/4.0    #6749      +/-   ##
===============================================
+ Coverage        69.64%   69.87%   +0.23%     
===============================================
  Files             1237     1367     +130     
  Lines           247617   256527    +8910     
  Branches         25436    26325     +889     
===============================================
+ Hits            172446   179247    +6801     
- Misses           68561    70376    +1815     
- Partials          6610     6904     +294     

Flag	Coverage Δ
Debug	69.87% <76.19%> (+0.23%)	:arrow_up:
production	64.72% <73.68%> (+0.53%)	:arrow_up:
test	88.89% <100.00%> (+<0.01%)	:arrow_up:

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

Files	Coverage Δ
src/Microsoft.ML.Core/CommandLine/CmdParser.cs	56.81% <100.00%> (+0.11%)	:arrow_up:
...oft.ML.Core/Utilities/FuncInstanceMethodInfo1`2.cs	90.32% <ø> (ø)
...oft.ML.Core/Utilities/FuncInstanceMethodInfo1`3.cs	80.64% <ø> (ø)
...oft.ML.Core/Utilities/FuncInstanceMethodInfo1`4.cs	80.64% <ø> (ø)
...oft.ML.Core/Utilities/FuncInstanceMethodInfo2`4.cs	80.64% <ø> (ø)
...oft.ML.Core/Utilities/FuncInstanceMethodInfo3`3.cs	0.00% <ø> (ø)
...oft.ML.Core/Utilities/FuncInstanceMethodInfo3`4.cs	0.00% <ø> (ø)
src/Microsoft.ML.Core/Utilities/Random.cs	80.12% <ø> (-5.13%)	:arrow_down:
src/Microsoft.ML.CpuMath/AvxIntrinsics.cs	96.68% <100.00%> (ø)
src/Microsoft.ML.CpuMath/SseIntrinsics.cs	95.24% <100.00%> (-1.11%)	:arrow_down:
... and 10 more

... and 142 files with indirect coverage changes