Reduce logging level to WARN while flattening a signature field without N entry

DEVSIX-4920

Author: Evgeniy Prudnikov

forms/src/main/java/com/itextpdf/forms/PdfAcroForm.java

@@ -721,7 +721,7 @@ public void flattenFields() {
                     }
                 }
             } else {
-                LOGGER.error(IoLogMessageConstant.N_ENTRY_IS_REQUIRED_FOR_APPEARANCE_DICTIONARY);
+                LOGGER.warn(FormsExceptionMessageConstant.N_ENTRY_IS_REQUIRED_FOR_APPEARANCE_DICTIONARY);
             }
 
             PdfArray fFields = getFields();

forms/src/main/java/com/itextpdf/forms/exceptions/FormsExceptionMessageConstant.java

@@ -34,5 +34,8 @@ public final class FormsExceptionMessageConstant {
     public static final String WRONG_FORM_FIELD_ADD_ANNOTATION_TO_THE_FIELD = "Wrong form field. Add annotation to the "
             + "field.";
 
+    public static final String N_ENTRY_IS_REQUIRED_FOR_APPEARANCE_DICTIONARY =
+            "\\N entry is required to be present in an appearance dictionary.";
+
     private FormsExceptionMessageConstant(){}
 }

forms/src/test/java/com/itextpdf/forms/FlatteningTest.java

@@ -42,6 +42,7 @@ This file is part of the iText (R) project.
  */
 package com.itextpdf.forms;
 
+import com.itextpdf.forms.exceptions.FormsExceptionMessageConstant;
 import com.itextpdf.io.logs.IoLogMessageConstant;
 import com.itextpdf.kernel.pdf.PdfDocument;
 import com.itextpdf.kernel.pdf.PdfReader;
@@ -83,7 +84,7 @@ public void flatteningFormFieldNoSubtypeInAPTest() throws IOException, Interrupt
     }
 
     @Test
-    @LogMessages(messages = @LogMessage(messageTemplate = IoLogMessageConstant.N_ENTRY_IS_REQUIRED_FOR_APPEARANCE_DICTIONARY))
+    @LogMessages(messages = @LogMessage(messageTemplate = FormsExceptionMessageConstant.N_ENTRY_IS_REQUIRED_FOR_APPEARANCE_DICTIONARY))
     public void formFlatteningTestWithoutNEntry() throws IOException, InterruptedException {
         String filename = "formFlatteningTestWithoutNEntry";
         String src = sourceFolder + filename + ".pdf";

io/src/main/java/com/itextpdf/io/logs/IoLogMessageConstant.java

@@ -370,6 +370,10 @@ public final class IoLogMessageConstant {
     public static final String NUM_TREE_SHALL_NOT_END_WITH_KEY =
             "Number tree ends with a key which is invalid according to the PDF specification.";
 
+    /**
+     * @deprecated Unused constant. Will be removed in 7.3
+     */
+    @Deprecated
     public static final String N_ENTRY_IS_REQUIRED_FOR_APPEARANCE_DICTIONARY =
             "\\N entry is required to be present in an appearance dictionary.";