GH-3608: Use Locale.ROOT for neutral, case insensitive comparisons

Fixes: #3608
Issue link: #3608

(cherry picked from commit 07cff76)

# Conflicts:
#	spring-kafka/src/main/java/org/springframework/kafka/annotation/KafkaListenerAnnotationBeanPostProcessor.java

spring-kafka/src/main/java/org/springframework/kafka/support/AbstractKafkaHeaderMapper.java

@@ -24,6 +24,7 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import java.util.stream.Collectors;
@@ -352,13 +353,13 @@ protected SimplePatternBasedHeaderMatcher(String pattern) {
 
 		SimplePatternBasedHeaderMatcher(String pattern, boolean negate) {
 			Assert.notNull(pattern, "Pattern must no be null");
-			this.pattern = pattern.toLowerCase();
+			this.pattern = pattern.toLowerCase(Locale.ROOT);
 			this.negate = negate;
 		}
 
 		@Override
 		public boolean matchHeader(String headerName) {
-			String header = headerName.toLowerCase();
+			String header = headerName.toLowerCase(Locale.ROOT);
 			if (PatternMatchUtils.simpleMatch(this.pattern, header)) {
 				LOGGER.debug(() ->
 						MessageFormat.format(