build(deps): bump io.undertow:undertow-core

Bumps [io.undertow:undertow-core](https://github.com/undertow-io/undertow) 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
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

samples/server/petstore/java-undertow/pom.xml

@@ -29,7 +29,7 @@
         <version.logback>1.5.19</version.logback>
         <version.junit>4.13.2</version.junit>
         <version.mockito>2.1.0-beta.124</version.mockito>
-        <version.undertow>2.3.20.Final</version.undertow>
+        <version.undertow>2.3.21.Final</version.undertow>
         <version.jsonpath>2.2.0</version.jsonpath>
         <version.httpclient>4.5.13</version.httpclient>
         <version.httpasyncclient>4.1.2</version.httpasyncclient>