Blazor common sample updates (dotnet#12848)

Author: guardrex

aspnetcore/blazor/common/samples/3.x/BlazorSample/App.razor

@@ -1 +1,5 @@
-<Router AppAssembly="typeof(Program).Assembly" />
+<Router AppAssembly="typeof(Program).Assembly">
+    <NotFoundContent>
+        <p>Sorry, there's nothing at this address.</p>
+    </NotFoundContent>
+</Router>

aspnetcore/blazor/common/samples/3.x/BlazorSample/BlazorSample.csproj

@@ -1,19 +1,15 @@
-<Project Sdk="Microsoft.NET.Sdk.Web">
+<Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
     <TargetFramework>netstandard2.0</TargetFramework>
-    <RestoreAdditionalProjectSources>
-      https://dotnet.myget.org/F/aspnetcore-dev/api/v3/index.json;
-      https://dotnet.myget.org/F/blazor-dev/api/v3/index.json;
-    </RestoreAdditionalProjectSources>
     <LangVersion>7.3</LangVersion>
     <RazorLangVersion>3.0</RazorLangVersion>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.AspNetCore.Blazor" Version="3.0.0-preview4-19216-03" />
-    <PackageReference Include="Microsoft.AspNetCore.Blazor.Build" Version="3.0.0-preview4-19216-03" PrivateAssets="all" />
-    <PackageReference Include="Microsoft.AspNetCore.Blazor.DevServer" Version="3.0.0-preview4-19216-03" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.AspNetCore.Blazor" Version="3.0.0-preview6.19307.2" />
+    <PackageReference Include="Microsoft.AspNetCore.Blazor.Build" Version="3.0.0-preview6.19307.2" PrivateAssets="all" />
+    <PackageReference Include="Microsoft.AspNetCore.Blazor.DevServer" Version="3.0.0-preview6.19307.2" PrivateAssets="all" />
   </ItemGroup>
 
 </Project>

aspnetcore/blazor/common/samples/3.x/BlazorSample/wwwroot/css/site.css

@@ -84,6 +84,18 @@ app {
     background-color: rgba(255, 255, 255, 0.1);
 }
 
+.valid.modified:not([type=checkbox]) {
+    outline: 1px solid #26b050;
+}
+
+.invalid {
+    outline: 1px solid red;
+}
+
+.validation-message {
+    color: red;
+}
+
 .panel {
     margin-bottom: 20px;
     border: 1px solid transparent;