Bump io.undertow:undertow-core (#455)

Bumps the maven group with 1 update in the /integration-test/fixtures/jar-app directory: [io.undertow:undertow-core](https://github.com/undertow-io/undertow).


Updates `io.undertow:undertow-core` from 2.3.20.Final to 2.3.21.Final
- [Release notes](https://github.com/undertow-io/undertow/releases)
- [Commits](undertow-io/undertow@2.3.20.Final...2.3.21.Final)

---
updated-dependencies:
- dependency-name: io.undertow:undertow-core
  dependency-version: 2.3.21.Final
  dependency-type: direct:production
  dependency-group: maven
...

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integration-test/fixtures/jar-app/pom.xml

@@ -21,7 +21,7 @@
     <dependency>
       <groupId>io.undertow</groupId>
       <artifactId>undertow-core</artifactId>
-      <version>2.3.20.Final</version>
+      <version>2.3.21.Final</version>
     </dependency>
   </dependencies>