build: bump org.openapitools:openapi-generator-maven-plugin in /example

Bumps org.openapitools:openapi-generator-maven-plugin from 7.20.0 to 7.21.0.

---
updated-dependencies:
- dependency-name: org.openapitools:openapi-generator-maven-plugin
  dependency-version: 7.21.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

example/rest-adapter/pom.xml

@@ -51,7 +51,7 @@
       <plugin>
         <groupId>org.openapitools</groupId>
         <artifactId>openapi-generator-maven-plugin</artifactId>
-        <version>7.20.0</version>
+        <version>7.21.0</version>
         <executions>
           <execution>
             <id>generate-open-api-source</id>