Compile System.Private.CoreLib with Crossgen2 by default (#44618)

After I deleted the Utf8String-related experimental code from the
runtime repo per JanK's suggestion, I can now enable Crossgen2
compilation for System.Private.CoreLib by default.

Thanks

Tomas

src/coreclr/crossgen-corelib.proj

@@ -5,7 +5,7 @@
   <Target Name="Build">
     <PropertyGroup>
       <!-- Default for using Crossgen2 when not set externally -->
-      <UseCrossgen2 Condition="'$(UseCrossgen2)' == ''">false</UseCrossgen2>
+      <UseCrossgen2 Condition="'$(UseCrossgen2)' == ''">true</UseCrossgen2>
 
       <OSPlatformConfig>$(TargetOS).$(TargetArchitecture).$(Configuration)</OSPlatformConfig>
       <RootBinDir>$(RepoRoot)\artifacts</RootBinDir>