Make the C# language check a warning instead of an error

Instead of reporting an error (both on BeforeCompile and CoreCompile) when we detect the project language isn't C# 9.0+, which breaks design-time builds (in addition to full builds) and causes bad IDE experience right-after install, make it a warning instead.

This gives the user the chance to build, perhaps attempt to type ThisAssembly and see that nothing happens, note the warning, and then cleanly uninstall the package.

Fixes #51

src/ThisAssembly.Prerequisites/ThisAssembly.Prerequisites.targets

@@ -6,12 +6,13 @@
             Condition="$([System.Version]::Parse('16.8.0').CompareTo($([System.Version]::Parse($(MSBuildVersion))))) == 1" />
 
     <!-- See https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/compiler-options/langversion-compiler-option -->
-    <Error Text="ThisAssembly uses Roslyn source generators, which are only supported in C# 9.0 or greater at the moment."
-           Condition="'$(Language)' != 'C#' OR 
-                ('$(LangVersion)' != 'preview' AND
-                 '$(LangVersion)' != 'latest' AND
-                 '$(LangVersion)' != 'latestMajor' AND
-                 '$(LangVersion)' &lt; '9.0')" />
+    <Warning Code="THIS001" 
+             Text="ThisAssembly uses Roslyn source generators, which are only supported in C# 9.0 or greater at the moment."
+             Condition="'$(Language)' != 'C#' OR 
+                  ('$(LangVersion)' != 'preview' AND
+                   '$(LangVersion)' != 'latest' AND
+                   '$(LangVersion)' != 'latestMajor' AND
+                   '$(LangVersion)' &lt; '9.0')" />
   </Target>
 
 </Project>