Add issue json property: merge_requests_count

Author: Yohann VANFRACHEM

src/GitLabApiClient/Models/Issues/Responses/Issue.cs

@@ -46,6 +46,9 @@ public sealed class Issue : ModifiableObject
         [JsonProperty("user_notes_count")]
         public int UserNotesCount { get; set; }
 
+        [JsonProperty("merge_requests_count")]
+        public int MergeRequestsCount { get; set; }
+
         [JsonProperty("state")]
         public IssueState State { get; set; }