Upgrade to Tomcat 8.5.6

Closes gh-7289

spring-boot-dependencies/pom.xml

@@ -176,7 +176,7 @@
 		<thymeleaf-layout-dialect.version>1.4.0</thymeleaf-layout-dialect.version>
 		<thymeleaf-extras-data-attribute.version>1.3</thymeleaf-extras-data-attribute.version>
 		<thymeleaf-extras-java8time.version>2.1.0.RELEASE</thymeleaf-extras-java8time.version>
-		<tomcat.version>8.5.5</tomcat.version>
+		<tomcat.version>8.5.6</tomcat.version>
 		<undertow.version>1.3.25.Final</undertow.version>
 		<velocity.version>1.7</velocity.version>
 		<velocity-tools.version>2.0</velocity-tools.version>