Upgrade to Tomcat 8.5.16

This commit uses the replacement for deprecated APIs in 8.5.16 to ease
a forward compatibility with Tomcat 9

Closes gh-9611

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/DefaultServletWebServerFactoryCustomizer.java

@@ -276,7 +276,7 @@ public class DefaultServletWebServerFactoryCustomizer
 					ProtocolHandler handler = connector.getProtocolHandler();
 					if (handler instanceof AbstractProtocol) {
 						AbstractProtocol<?> protocol = (AbstractProtocol<?>) handler;
-						protocol.setBacklog(acceptCount);
+						protocol.setAcceptCount(acceptCount);
 					}
 				}
 

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/servlet/DefaultServletWebServerFactoryCustomizerTests.java

@@ -352,7 +352,7 @@ public class DefaultServletWebServerFactoryCustomizerTests {
 		embeddedFactory.start();
 		try {
 			assertThat(((AbstractProtocol<?>) embeddedFactory.getTomcat().getConnector()
-					.getProtocolHandler()).getBacklog()).isEqualTo(10);
+					.getProtocolHandler()).getAcceptCount()).isEqualTo(10);
 		}
 		finally {
 			embeddedFactory.stop();