Make TestClassScanner package private again

TestClassScanner was made public with the assumption that Spring Boot's
AOT testing support might need to work directly with that class; however,
it turns out that Spring Boot does not currently have such a need.

In light of that, this commit restores TestClassScanner's visibility to
package private.

spring-test/src/main/java/org/springframework/test/context/aot/TestClassScanner.java

@@ -79,7 +79,7 @@ import static org.springframework.core.annotation.MergedAnnotations.SearchStrate
  * @author Sam Brannen
  * @since 6.0
  */
-public class TestClassScanner {
+class TestClassScanner {
 
 	// JUnit Jupiter
 	private static final String EXTEND_WITH_ANNOTATION_NAME = "org.junit.jupiter.api.extension.ExtendWith";