Mark backscompat package as beta before publishing

Since it doesn't use the same versioning scheme as the main package (because it will remain fixed at the time we deprecate the version), we must update the version manually.

src/Attributed/Attributed.msbuildproj

@@ -4,7 +4,7 @@
     <TargetFramework>net6.0</TargetFramework>
     <PackageId>Devlooped.Extensions.DependencyInjection.Attributed</PackageId>
     <Description>Superseded by Devlooped.Extensions.DependencyInjection</Description>
-    <Version>2.1.0</Version>
+    <Version>2.1.0-beta</Version>
   </PropertyGroup>
 
   <ItemGroup>