Support ancient .NET in Filter

SabreTools.Filter/FilterRunner.cs

@@ -12,7 +12,11 @@ namespace SabreTools.Filter
         /// <summary>
         /// Set of filters to be run against an object
         /// </summary>
+#if NETFRAMEWORK || NETCOREAPP3_1
+        public FilterObject[] Filters { get; private set; }
+#else
         public FilterObject[] Filters { get; init; }
+#endif
 
         public FilterRunner(FilterObject[]? filters)
         {
@@ -35,7 +39,7 @@ namespace SabreTools.Filter
 
             this.Filters = filters.ToArray();
         }
-    
+
         /// <summary>
         /// Run filtering on a DictionaryBase item
         /// </summary>