Remove Microbenchmarks .NET 6 hard-coded toolchain

[AndreReise]

I've tried to run the microbenchmarks locally and ran into the following error while generating the BDN boilerplate code:

 Standard error:
   Determining projects to restore...
C:\Personal\Repositories\grpc-dotnet\perf\Grpc.AspNetCore.Microbenchmarks\bin\Release\net10.0\Grpc.AspNetCore.Microbenchmarks-1\BenchmarkDotNet.Autogenerated.csproj : error NU1201: Project Grpc.AspNetCore.Microbenchmarks is not compatible with net6.0 (.NETCoreApp,Version=v6.0). Project Grpc.AspNetCore.Microbenchmarks supports: net10.0 (.NETCoreApp,Version=v10.0)
  Restored C:\Personal\Repositories\grpc-dotnet\src\Grpc.Core.Api\Grpc.Core.Api.csproj (in 173 ms).
  Restored C:\Personal\Repositories\grpc-dotnet\src\Grpc.Net.Common\Grpc.Net.Common.csproj (in 173 ms).
  Restored C:\Personal\Repositories\grpc-dotnet\src\Grpc.Net.Client\Grpc.Net.Client.csproj (in 177 ms).
  Restored C:\Personal\Repositories\grpc-dotnet\src\Grpc.AspNetCore.Server\Grpc.AspNetCore.Server.csproj (in 167 ms).
  Restored C:\Personal\Repositories\grpc-dotnet\src\Grpc.AspNetCore.Server.ClientFactory\Grpc.AspNetCore.Server.ClientFactory.csproj (in 167 ms).
  Restored C:\Personal\Repositories\grpc-dotnet\src\Grpc.AspNetCore\Grpc.AspNetCore.csproj (in 182 ms).
  Restored C:\Personal\Repositories\grpc-dotnet\src\Grpc.Net.ClientFactory\Grpc.Net.ClientFactory.csproj (in 184 ms).
  Failed to restore C:\Personal\Repositories\grpc-dotnet\perf\Grpc.AspNetCore.Microbenchmarks\bin\Release\net10.0\Grpc.AspNetCore.Microbenchmarks-1\BenchmarkDotNet.Autogenerated.csproj (in 148 ms).
  Restored C:\Personal\Repositories\grpc-dotnet\perf\Grpc.AspNetCore.Microbenchmarks\Grpc.AspNetCore.Microbenchmarks.csproj (in 200 ms).

// BenchmarkDotNet has failed to build the auto-generated boilerplate code.
// It can be found in C:\Personal\Repositories\grpc-dotnet\perf\Grpc.AspNetCore.Microbenchmarks\bin\Release\net10.0\Grpc.AspNetCore.Microbenchmarks-1
// Please follow the troubleshooting guide: https://benchmarkdotnet.org/articles/guides/troubleshooting.html

For the context, I don't have .NET6 runtime/sdk installed.

The issue is that the .NET 6 toolchain is hard-coded here, which was introduced in da6dcc5. I suppose it was done because at that time BDN didn’t support .NET 6 targets.

This issue can be worked around by running benchmarks specifying a concrete runtime, e.g.: dotnet run -c Release -- --runtimes net10.0

However, it still outputs broken .NET 6 target results:

How about removing that hard-coded toolchain target?