Use toLowerCase() and toUpperCase() with Locale.ENGLISH

This commit updates all `toLowerCase()` and `toUpperCase` invocations to
use the variant that takes a `Locale` to avoid locale-specific side
effect.

Closes gh-12213

spring-boot-integration-tests/spring-boot-launch-script-tests/src/test/java/org/springframework/boot/launchscript/SysVinitLaunchScriptIT.java

@@ -24,6 +24,7 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashSet;
 import java.util.List;
+import java.util.Locale;
 import java.util.concurrent.TimeUnit;
 import java.util.regex.Pattern;
 
@@ -306,7 +307,8 @@ public class SysVinitLaunchScriptIT {
 	private String buildImage(DockerClient docker) {
 		String dockerfile = "src/test/resources/conf/" + this.os + "/" + this.version
 				+ "/Dockerfile";
-		String tag = "spring-boot-it/" + this.os.toLowerCase() + ":" + this.version;
+		String tag = "spring-boot-it/" + this.os.toLowerCase(Locale.ENGLISH) + ":"
+				+ this.version;
 		BuildImageResultCallback resultCallback = new BuildImageResultCallback() {
 
 			private List<BuildResponseItem> items = new ArrayList<BuildResponseItem>();