Consistently synchronize on this.monitor in InMemoryAuditEventRepository

See gh-6261

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/audit/InMemoryAuditEventRepository.java

@@ -82,7 +82,7 @@ public class InMemoryAuditEventRepository implements AuditEventRepository {
 	@Override
 	public List<AuditEvent> find(String principal, Date after, String type) {
 		LinkedList<AuditEvent> events = new LinkedList<AuditEvent>();
-		synchronized (this.events) {
+		synchronized (this.monitor) {
 			for (int i = 0; i < this.events.length; i++) {
 				AuditEvent event = resolveTailEvent(i);
 				if (event != null && isMatch(principal, after, type, event)) {