Deprecate JUnit 4 OutputCaptureRule

Closes gh-47256

core/spring-boot-test/src/main/java/org/springframework/boot/test/system/OutputCaptureRule.java

@@ -50,7 +50,9 @@ import static org.hamcrest.Matchers.allOf;
  * @author Phillip Webb
  * @author Andy Wilkinson
  * @since 2.2.0
+ * @deprecated since 4.0.0 in favor of JUnit 5 and {@link OutputCaptureExtension}
  */
+@Deprecated
 public class OutputCaptureRule implements TestRule, CapturedOutput {
 
 	private final OutputCapture delegate = new OutputCapture();

core/spring-boot-test/src/test/java/org/springframework/boot/test/system/OutputCaptureRuleTests.java

@@ -26,6 +26,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Roland Weisleder
  */
+@SuppressWarnings("deprecation")
 public class OutputCaptureRuleTests {
 
 	@Rule