build(deps): bump System.Collections.Immutable in /Sample (#539)

Bumps [System.Collections.Immutable](https://github.com/dotnet/runtime) from 5.0.0 to 6.0.0.
- [Release notes](https://github.com/dotnet/runtime/releases)
- [Commits](dotnet/runtime@v5.0.0...v6.0.0)

---
updated-dependencies:
- dependency-name: System.Collections.Immutable
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Sample/SextantSample/SextantSample.csproj

@@ -4,7 +4,7 @@
         <TargetFramework>netstandard2.0</TargetFramework>
     </PropertyGroup>
     <ItemGroup>
-        <PackageReference Include="System.Collections.Immutable" Version="5.0.0" />
+        <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
         <PackageReference Include="Pharmacist.MsBuild" Version="2.*" PrivateAssets="all" />
         <PackageReference Include="Pharmacist.Common" Version="2.*" />
     </ItemGroup>

src/Sextant.XamForms/Sextant.XamForms.csproj

@@ -11,7 +11,7 @@
 
   <ItemGroup>
     <PackageReference Include="ReactiveUI.XamForms" Version="17.*" />
-    <PackageReference Include="System.Collections.Immutable" Version="5.0.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
   </ItemGroup>
 
   <ItemGroup>

src/Sextant/Sextant.csproj

@@ -12,7 +12,7 @@
   <ItemGroup>
     <Compile Remove="Platforms\**\*.cs" />
     <None Include="Platforms\**\*.cs" />
-    <PackageReference Include="System.Collections.Immutable" Version="5.0.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
     <PackageReference Include="ReactiveUI" Version="17.*" />
   </ItemGroup>