Support trailing comment in DTD declaration in XML config

Prior to this commit, Spring failed to determine that an XML config file
was DTD-based if the DTD declaration was followed by a comment.

This commit fixes this by modifying the consumeCommentTokens(String)
algorithm in XmlValidationModeDetector so that both leading and trailing
comments are properly consumed without losing any XML content.

Closes gh-23605

spring-core/src/test/resources/org/springframework/util/xml/dtdWithCommentOnNextLine.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "https://www.springframework.org/dtd/spring-beans-2.0.dtd">
+<!-- comment on next line -->
+
+<beans>
+
+</beans>

spring-core/src/test/resources/org/springframework/util/xml/dtdWithLeadingComment.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- leading comment --><!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "https://www.springframework.org/dtd/spring-beans-2.0.dtd">
+
+<beans>
+
+</beans>

spring-core/src/test/resources/org/springframework/util/xml/dtdWithMultipleComments.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- comment #1 -->  <!-- comment #2 -->  <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "https://www.springframework.org/dtd/spring-beans-2.0.dtd"><!--
+trailing  
+      comment
+-->
+
+<beans>
+
+</beans>

spring-core/src/test/resources/org/springframework/util/xml/dtdWithTrailingComment.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "https://www.springframework.org/dtd/spring-beans-2.0.dtd"><!-- trailing comment -->
+
+<beans>
+
+</beans>