Upgrade to Undertow 2.0.21.Final

Closes gh-17176

spring-boot-project/spring-boot-dependencies/pom.xml

@@ -204,7 +204,7 @@
 		<thymeleaf-extras-java8time.version>3.0.4.RELEASE</thymeleaf-extras-java8time.version>
 		<tomcat.version>9.0.20</tomcat.version>
 		<unboundid-ldapsdk.version>4.0.11</unboundid-ldapsdk.version>
-		<undertow.version>2.0.20.Final</undertow.version>
+		<undertow.version>2.0.21.Final</undertow.version>
 		<webjars-hal-browser.version>3325375</webjars-hal-browser.version>
 		<webjars-locator-core.version>0.37</webjars-locator-core.version>
 		<woodstox.version>5.0.3</woodstox.version>