Pin version variables to the ASP.NET Core 2.1.2 baseline

This reverts our previous policy of cascading versions on all servicing updates.
This moves variables into the 'pinned' section, and points them to the latest
stable release (versions that were used at the time of the 2.1.2 release).

build/dependencies.props

@@ -4,7 +4,7 @@
   </PropertyGroup>
 
   <!-- These package versions may be overridden or updated by automation. -->
-  <PropertyGroup Label="Package Versions: Auto" />
+  <PropertyGroup Label="Package Versions: Auto">
     <AngleSharpPackageVersion>0.9.9</AngleSharpPackageVersion>
     <DotNetEfPackageVersion>2.1.1</DotNetEfPackageVersion>
     <InternalAspNetCoreSdkPackageVersion>2.1.3-rtm-15802</InternalAspNetCoreSdkPackageVersion>