chore(deps): update dependency system.reactive to v6 (#905)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

src/Akavache.CosmosDB/Akavache.CosmosDB.csproj

@@ -11,7 +11,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Reactive" Version="4.4.1" />
+    <PackageReference Include="System.Reactive" Version="6.0.0" />
     <PackageReference Include="Splat" Version="9.*" />
     <PackageReference Include="Microsoft.Azure.Cosmos" Version="3.8.0" />
   </ItemGroup>