build(deps): bump System.Collections.Immutable from 1.5.0 to 1.6.0 (#2211)

Bumps [System.Collections.Immutable](https://github.com/dotnet/corefx) from 1.5.0 to 1.6.0.
- [Release notes](https://github.com/dotnet/corefx/releases)
- [Commits](https://github.com/dotnet/corefx/commits)

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

Author: dependabot-preview[bot]

src/ReactiveUI.Fody.Analyzer/ReactiveUI.Fody.Analyzer.csproj

@@ -12,7 +12,7 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.CodeAnalysis.Analyzers" Version="2.9.3" PrivateAssets="all" />
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" Version="3.2.1" PrivateAssets="all" />
-    <PackageReference Include="System.Collections.Immutable" Version="1.5.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="1.6.0" />
   </ItemGroup>
 
   <ItemGroup>