Try fixing failing CI build.

Author: OlgaMaciaszek

spring-cloud-commons/src/main/java/org/springframework/cloud/configuration/TlsProperties.java

@@ -16,6 +16,7 @@
 
 package org.springframework.cloud.configuration;
 
+import java.util.Locale;
 import java.util.Map;
 
 import org.springframework.core.io.Resource;
@@ -143,7 +144,7 @@ private String fileExtensionOf(Resource resource) {
 		String name = resource.getFilename();
 		int index = name.lastIndexOf('.');
 
-		return index < 0 ? "" : name.substring(index + 1).toLowerCase();
+		return index < 0 ? "" : name.substring(index + 1).toLowerCase(Locale.ROOT);
 	}
 
 }

src/checkstyle/checkstyle-suppressions.xml

@@ -22,4 +22,6 @@
 	<suppress files=".*AutoConfiguration.*" checks="HideUtilityClassConstructor"/>
 	<suppress files=".*AutoConfiguration.*" checks="FinalClass"/>
 	<suppress files=".*ReactiveDiscoveryClient.*" checks="JavadocVariable"/>
+	<suppress files="[\\/]src[\\/](test|testFixtures)[\\/](java|java21)[\\/]" checks="RegexpSinglelineJava" id="toLowerCaseWithoutLocale"/>
+	<suppress files="[\\/]src[\\/](test|testFixtures)[\\/](java|java21)[\\/]" checks="RegexpSinglelineJava" id="toUpperCaseWithoutLocale"/>
 </suppressions>