fix: update integration workflow, enforcer rule, and Java badge for Java 21

- Point integration workflow to soul2zimate/exhort-integration-tests
  which has Java 21 configured
- Update Maven Enforcer requireJavaVersion from [11,) to [21,)
- Update Java version badge in CONTRIBUTING.md from 11 to 21

Implements TC-3969

Assisted-by: Claude Code

Author: soul2zimate

CONTRIBUTING.md

@@ -113,4 +113,4 @@ contribution. See the [DCO](DCO) file for details.
 [1]: https://github.com/guacsec/trustify-da-api-spec/blob/main/api/v5/openapi.yaml
 
 <!-- Badge links -->
-[10]: https://badgen.net/badge/Java%20Version/11/5382a1
+[10]: https://badgen.net/badge/Java%20Version/21/5382a1

pom.xml

@@ -628,7 +628,7 @@ limitations under the License.]]>
           <rules>
             <dependencyConvergence />
             <requireJavaVersion>
-              <version>[11,)</version>
+              <version>[21,)</version>
             </requireJavaVersion>
             <requireMavenVersion>
               <version>[3.8,)</version>