v10: Cleanup project files (#13111)

* Backport project cleanup from #12907

* Remove empty Directory.Build.props

* Fix GenerateAppsettingsSchema target

* Re-add empty Directory.Build.props to prevent inheritance

* Add NRT to-do comment

* Explicitly generate HTTPS development certificate for E2E test

tests/Directory.Build.props

@@ -2,6 +2,11 @@
   <!-- Enable multi-level merging -->
   <Import Project="$([MSBuild]::GetPathOfFileAbove('Directory.Build.props', '$(MSBuildThisFileDirectory)../'))" />
 
+  <PropertyGroup>
+    <!-- TODO: Update test projects to use nullable reference types and remove this to enable nullable warnings as errors (the solution default) -->
+    <Nullable>annotations</Nullable>
+  </PropertyGroup>
+  
   <!-- Specify rule set for all test projects -->
   <PropertyGroup>
     <CodeAnalysisRuleSet>$(MSBuildThisFileDirectory)codeanalysis.ruleset</CodeAnalysisRuleSet>