Bump BenchmarkDotNet from 0.11.5 to 0.12.0 in /Rx.NET/Source

Bumps [BenchmarkDotNet](https://github.com/dotnet/BenchmarkDotNet) from 0.11.5 to 0.12.0.
- [Release notes](https://github.com/dotnet/BenchmarkDotNet/releases)
- [Commits](https://github.com/dotnet/BenchmarkDotNet/compare/v0.11.5...v0.12.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Rx.NET/Source/benchmarks/Benchmarks.System.Reactive/Benchmarks.System.Reactive.csproj

@@ -21,7 +21,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="BenchmarkDotNet" Version="0.11.5" />
+    <PackageReference Include="BenchmarkDotNet" Version="0.12.0" />
     <PackageReference Include="WindowsBase" Version="4.6.1055" />
   </ItemGroup>