Cleanup unused dependencies (#148)

Solution compiles fine without these direct references:
```xml
<PackageReference Include="System.Buffers" Version="4.5.1" />
<PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" />
<PackageReference Include="System.Collections.Immutable" Version="8.0.0" />
```
Most already comes from
[System.Text.Json](https://www.nuget.org/packages/System.Text.Json/8.0.5):
-
[Microsoft.Bcl.AsyncInterfaces](https://www.nuget.org/packages/Microsoft.Bcl.AsyncInterfaces/)
(>= 8.0.0)
- [System.Buffers](https://www.nuget.org/packages/System.Buffers/) (>=
4.5.1)
- [System.Memory](https://www.nuget.org/packages/System.Memory/) (>=
4.5.5)
-
[System.Runtime.CompilerServices.Unsafe](https://www.nuget.org/packages/System.Runtime.CompilerServices.Unsafe/)
(>= 6.0.0)
-
[System.Text.Encodings.Web](https://www.nuget.org/packages/System.Text.Encodings.Web/)
(>= 8.0.0)
-
[System.Threading.Tasks.Extensions](https://www.nuget.org/packages/System.Threading.Tasks.Extensions/)
(>= 4.5.4)

So the actual cleanup is only `System.Collections.Immutable`.

src/Elastic.Transport/Elastic.Transport.csproj

@@ -30,15 +30,9 @@
   </ItemGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' != 'net8.0'">
-    <PackageReference Include="System.Threading.Tasks.Extensions" Version="4.5.4" />
-    <PackageReference Include="System.Buffers" Version="4.5.1" />
     <PackageReference Include="Microsoft.CSharp" Version="4.7.0" />
-  </ItemGroup>
-
-  <ItemGroup Condition="'$(TargetFramework)' != 'net8.0'">
     <PackageReference Include="System.Text.Json" Version="8.0.5" />
     <PackageReference Include="System.Diagnostics.DiagnosticSource" Version="8.0.0" />
-    <PackageReference Include="System.Collections.Immutable" Version="8.0.0" />
   </ItemGroup>
 
   <ItemGroup>