Refactor System.Text.Json deps: remove for net6/net8, update netstandard to 8.0.5 (#17)

Author: albertospelta

src/Dax.Formatter/Dax.Formatter.csproj

@@ -45,8 +45,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="System.Text.Json" Version="6.0.11" Condition="'$(TargetFramework)' == 'net6.0' or '$(TargetFramework)' == 'netstandard2.0'" />
-    <PackageReference Include="System.Text.Json" Version="8.0.5" Condition="'$(TargetFramework)' == 'net8.0'" />
+    <PackageReference Include="System.Text.Json" Version="8.0.5" Condition="'$(TargetFramework)' == 'netstandard2.0'" />
   </ItemGroup>
 
   <PropertyGroup>