build(deps): bump system.collections.immutable from 1.5.0 to 1.6.0 (#64)

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] <support@dependabot.com>

src/Pharmacist.Core/Pharmacist.Core.csproj

@@ -17,7 +17,7 @@
     <PackageReference Include="NuGet.LibraryModel" Version="5.2.0" />
     <PackageReference Include="polly" Version="7.1.1" />
     <PackageReference Include="splat" Version="9.0.5" />
-    <PackageReference Include="system.collections.immutable" Version="1.5.0" />
+    <PackageReference Include="system.collections.immutable" Version="1.6.0" />
     <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.5.2" />
   </ItemGroup>