Redmine 6 compatibility: update patched method

Author: nanego

lib/redmine_datetime_custom_field/query_patch.rb

@@ -63,7 +63,7 @@ def date_clause(table, field, from, to, is_custom_filter)
       else
         from = from - 1 # second
       end
-      if self.class.default_timezone == :utc
+      if ActiveRecord.default_timezone == :utc
         from = from.utc
       end
 
@@ -84,7 +84,7 @@ def date_clause(table, field, from, to, is_custom_filter)
       if to.is_a?(Date)
         to = date_for_user_time_zone(to.year, to.month, to.day).end_of_day
       end
-      if self.class.default_timezone == :utc
+      if ActiveRecord.default_timezone == :utc
         to = to.utc
       end