fix: add explicit namespace usings to resolve compile errors

Author: pandinocoder

Intersect.Server/Web/Controllers/AvatarController.cs

@@ -5,7 +5,9 @@
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.OutputCaching;
+using SixLabors.ImageSharp;
 using SixLabors.ImageSharp.Formats;
+using SixLabors.ImageSharp.Processing;
 
 namespace Intersect.Server.Web.Controllers;