Bump Microsoft.AspNetCore.All in /EFCore/EFCoreSamples/MigrationsWebApp

Bumps [Microsoft.AspNetCore.All](https://github.com/aspnet/AspNetCore) from 2.1.15 to 2.1.21.
- [Release notes](https://github.com/aspnet/AspNetCore/releases)
- [Changelog](https://github.com/dotnet/aspnetcore/blob/main/docs/ReleasePlanning.md)
- [Commits](dotnet/aspnetcore@v2.1.15...v2.1.21)

---
updated-dependencies:
- dependency-name: Microsoft.AspNetCore.All
  dependency-type: direct:production
...

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Author: dependabot[bot]

EFCore/EFCoreSamples/MigrationsWebApp/MigrationsWebApp.csproj

@@ -1,11 +1,11 @@
-<Project Sdk="Microsoft.NET.Sdk.Web">
+<Project Sdk="Microsoft.NET.Sdk.Web">
 
   <PropertyGroup>
     <TargetFramework>netcoreapp2.1</TargetFramework>
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.AspNetCore.All" Version="2.1.15" />
+    <PackageReference Include="Microsoft.AspNetCore.All" Version="2.1.21" />
   </ItemGroup>
 
   <ItemGroup>