Add 9.0 TFM to Microbenchmarks

[CarnaViire]

This is an attempt to add net9.0 TFM to microbenchmarks csproj, otherwise it is impossible to run them for .NET 9 in the way it's suggested by src/benchmarks/micro/README.md:

➜ dotnet run -c release -f net9.0

C:\Users\knatalia\dev\git\tmp\performance\.dotnet\sdk\9.0.100-alpha.1.23572.8\Sdks\Microsoft.NET.Sdk\targets\Microsoft.PackageDependencyResolution.targets(266,5): error NETSDK1005: Assets file
 'C:\Users\knatalia\dev\git\tmp\performance\artifacts\obj\MicroBenchmarks\project.assets.json' doesn't have a target for 'net9.0'. Ensure that restore has run and that you have included 'net9.
0' in the TargetFrameworks for your project. [C:\Users\knatalia\dev\git\tmp\performance\src\benchmarks\micro\MicroBenchmarks.csproj::TargetFramework=net9.0]

The build failed. Fix the build errors and run again.

Contributes to dotnet/crank#662

[CarnaViire]

CI failures are unrelated (they are the same as in e.g. #3495)

[cincuranet]

Thanks!