fix(MailParams): make Placeholders Dictionary

EnvelopeGenerator.Application/Configurations/MailParams.cs

@@ -1,6 +1,4 @@
-using System.Collections.Immutable;
+namespace EnvelopeGenerator.Application.Configurations;
-
-namespace EnvelopeGenerator.Application.Configurations;
 
 /// <summary>
 /// 
@@ -10,5 +8,5 @@ public class MailParams
     /// <summary>
     /// 
     /// </summary>
-    public required ImmutableDictionary<string, string> Placeholders { get; init; }
+    public required Dictionary<string, string> Placeholders { get; init; }
 }