adds related object to rack reservation changelogs (#19306)

Author: ITJamie

netbox/dcim/models/racks.py

@@ -725,3 +725,8 @@ def clean(self):
     @property
     def unit_list(self):
         return array_to_string(self.units)
+
+    def to_objectchange(self, action):
+        objectchange = super().to_objectchange(action)
+        objectchange.related_object = self.rack
+        return objectchange