Refactored pre spt mod loader to user IOnLoad, moved Watermark to IOnLoad as well (#313)

Co-authored-by: Alex <alex@dm-me-for-questions.com>

Libraries/SPTarkov.Server.Core/Models/External/IPostDBLoadModAsync.cs

@@ -1,5 +1,20 @@
 namespace SPTarkov.Server.Core.Models.External;
 
+/// <summary>
+/// This interface used to be used in TS to load mods after the database finished loading.
+/// This class is now deprecated and should not be used, see code example below for replacement.
+/// </summary>
+/// <code>
+/// [Injectable(TypePriority = OnLoadOrder.Database + 1)]
+/// public class MyMod : IOnLoad
+/// {
+///   // ... implementation
+/// }
+/// </code>
+/// <remarks>
+/// <b>DEPRECATED, see code example above for replacement!</b>
+/// </remarks>
+[Obsolete("This interface is obsolete and will be removed in 4.1.0, please use IOnLoad instead with the desired Injectable(TypePriority). See class documentation for examples.")]
 public interface IPostDBLoadModAsync
 {
     Task PostDBLoadAsync();