Avoid usage of StringBuffer where possible

See gh-32519

spring-boot-project/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/properties/AnnotationsPropertySource.java

@@ -111,7 +111,7 @@ public class AnnotationsPropertySource extends EnumerablePropertySource<Class<?>
 
 	private String toKebabCase(String name) {
 		Matcher matcher = CAMEL_CASE_PATTERN.matcher(name);
-		StringBuffer result = new StringBuffer();
+		StringBuilder result = new StringBuilder();
 		while (matcher.find()) {
 			matcher.appendReplacement(result, matcher.group(1) + '-' + StringUtils.uncapitalize(matcher.group(2)));
 		}

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/DefaultLaunchScript.java

@@ -88,7 +88,7 @@ public class DefaultLaunchScript implements LaunchScript {
 	}
 
 	private String expandPlaceholders(String content, Map<?, ?> properties) throws IOException {
-		StringBuffer expanded = new StringBuffer();
+		StringBuilder expanded = new StringBuilder();
 		Matcher matcher = PLACEHOLDER_PATTERN.matcher(content);
 		while (matcher.find()) {
 			String name = matcher.group(1);