Removed C# 9.0 support for Host project

The `OnTopic.AspNetCore.Mvc.Host` project builds fine locally, but won't build in Azure DevOps as part of the CI/CD pipeline. To attempt a fix, I'm reverting the host project—and only the host project—back to the default of C# 8.0.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc.Host/OnTopic.AspNetCore.Mvc.Host.csproj

@@ -1,10 +1,9 @@
-<Project Sdk="Microsoft.NET.Sdk.Web">
+<Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
     <TargetFramework>netcoreapp3.0</TargetFramework>
     <UserSecretsId>62eb85bf-f802-4afd-8bec-3d344e1cfc79</UserSecretsId>
     <IsPackable>false</IsPackable>
-    <LangVersion>9.0</LangVersion>
   </PropertyGroup>
 
   <ItemGroup>

OnTopic.AspNetCore.Mvc.Host/Views/Layout/_Layout.cshtml

@@ -26,7 +26,7 @@
       <!-- /Page Header Area -->
 
       <article itemscope itemtype="http://schema.org/WebPageElement" itemprop="mainContentOfPage" class="grid-container">
-        <h1>@(Model.ContentType is "PageGroup"? "Overview" : Model.Title)</h1>
+        <h1>@(Model.ContentType == "PageGroup"? "Overview" : Model.Title)</h1>
         <p class="subtitle">@Model.Subtitle</p>
         <section class="body">
           <partial name="_TopicAttributes" />

OnTopic.AspNetCore.Mvc.Host/Views/Shared/Components/PageLevelNavigation/Default.cshtml

@@ -2,7 +2,7 @@
 
 <h2>PageLevelNavigation</h2>
 <ul>
-  @if (Model.NavigationRoot is not null) {
+  @if (Model.NavigationRoot != null) {
     foreach (var topic in Model.NavigationRoot.Children) {
       <li><a href="@topic.WebPath">@(topic.ShortTitle?? topic.Title)</a></li>
     }