Upgrade to Undertow 1.3.26.Final

Closes gh-8079

Author: wilkinsona

spring-boot-dependencies/pom.xml

@@ -177,7 +177,7 @@
 		<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>8.5.11</tomcat.version>
-		<undertow.version>1.3.25.Final</undertow.version>
+		<undertow.version>1.3.26.Final</undertow.version>
 		<velocity.version>1.7</velocity.version>
 		<velocity-tools.version>2.0</velocity-tools.version>
 		<webjars-hal-browser.version>9f96c74</webjars-hal-browser.version>