Remove DisposableExtensions and update System.Reactive (#87)

* Remove DisposableExtensions and update System.Reactive

Deleted the custom DisposableExtensions class and updated the System.Reactive package to version 6.1.0. Updated tests to use System.Reactive.Disposables.Fluent, reflecting the migration to built-in extension methods.

* Update README.md

Author: ChrisPulman

README.md

@@ -278,6 +278,7 @@ Issues / PRs welcome. Please keep additions dependency
 - Added liveness operators (`Heartbeat`, `DetectStale`, `BufferUntilInactive`).
 - Added resilience (`RetryWithBackoff`, expanded `OnErrorRetry` overloads).
 - Added flow control (`Conflate`, `ThrottleFirst`, `DebounceImmediate`).
+- Removed DisposeWith extension use System.Reactive.Disposables.Fluent from System.Reactive.
 
 ---
 Happy reactive coding!

src/ReactiveUI.Extensions.Tests/DisposableExtensionsTests.cs

@@ -4,6 +4,7 @@
 
 using System;
 using System.Reactive.Disposables;
+using System.Reactive.Disposables.Fluent;
 using FluentAssertions;
 using Xunit;
 

src/ReactiveUI.Extensions/DisposableExtensions.cs

@@ -15,7 +15,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageVersion Include="System.Reactive" Version="6.0.2" />
+    <PackageVersion Include="System.Reactive" Version="6.1.0" />
   </ItemGroup>
 
   <ItemGroup>