Upgrade to Tomcat 9.0.21

Closes gh-17182

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

@@ -202,7 +202,7 @@
 		<thymeleaf-layout-dialect.version>2.4.1</thymeleaf-layout-dialect.version>
 		<thymeleaf-extras-data-attribute.version>2.0.1</thymeleaf-extras-data-attribute.version>
 		<thymeleaf-extras-java8time.version>3.0.4.RELEASE</thymeleaf-extras-java8time.version>
-		<tomcat.version>9.0.20</tomcat.version>
+		<tomcat.version>9.0.21</tomcat.version>
 		<unboundid-ldapsdk.version>4.0.11</unboundid-ldapsdk.version>
 		<undertow.version>2.0.21.Final</undertow.version>
 		<webjars-hal-browser.version>3325375</webjars-hal-browser.version>