Bump System.Reactive from 5.0.0 to 6.0.0

Bumps [System.Reactive](https://github.com/dotnet/reactive) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/dotnet/reactive/releases)
- [Commits](https://github.com/dotnet/reactive/commits)

---
updated-dependencies:
- dependency-name: System.Reactive
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

src/SerialPortRx/SerialPortRx.csproj

@@ -1,12 +1,12 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0;net6.0;net7.0;net6.0-windows10.0.17763.0;net7.0-windows10.0.17763.0</TargetFrameworks>
   </PropertyGroup>
   <PropertyGroup Condition="'$(TargetFramework)'=='net6.0-windows10.0.17763.0' or '$(TargetFramework)'=='net7.0-windows10.0.17763.0'">
     <DefineConstants>HasWindows</DefineConstants>
   </PropertyGroup>
   <ItemGroup>
-    <PackageReference Include="System.Reactive" Version="5.0.0" />
+    <PackageReference Include="System.Reactive" Version="6.0.0" />
     <PackageReference Include="System.IO.Ports" Version="7.0.0" />
   </ItemGroup>