.NET projects cleanup + set minimum dependency to net472 for microbenchmarks

[caaavik-msft]

This PR does a few things:

• Resolves all warnings in our .NET projects (excluding those in src/scenarios).
  • This includes the NU1510 warnings reported in MicroBenchmarks build failing with error NU1510 #4975.
• Updates the minimum dependency of Microbenchmarks from .NET 4.6.2 to .NET 4.7.2, which allows us to safely target netstandard2.0 in BenchmarkDotNet.Extensions without requiring an additional net462 target.
  • I have removed the net462 job from the yml files, if we need automated .NET Framework runs in the future, we will need to revisit it anyways
• Enabled nullable for all the projects and updated the code accordingly
• Disabled NETSDK1138 which was being reported every time because we build for older .NET versions
• Updated the .NET framework conditioning logic in the MSBuild project files to use the new TargetFramework functions
• Enabled ProduceReferenceAssembly in the netstandard2.0 projects so that we can have incremental compilation working. Without this, every time a change is made to BenchmarkDotNet.Extensions or Reporting it would recompile Microbenchmarks.

[xtqqczze]

Many conditions can be simplified, like in #4977

[LoopedBard3]

Looks good to me, if we want to include the extra cleanup in the MicroBenchmarks.csproj file in this PR, that works for me, or we can do it in another PR.