Migrate patch-diff-report-tool to Version 21

The patch-diff-report-tool in the contribution repo is configured for Java 17 (in its pom.xml)
https://github.com/checkstyle/contribution/blob/master/patch-diff-report-tool/pom.xml

but the Checkstyle master branch now requires Java 21.

I ran Github, generate report and it fails due to version mismatch. Logs attached here:

```
Error: Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:3.6.2:enforce (enforce-versions) on project checkstyle:
Error: Rule 0: org.apache.maven.enforcer.rules.version.RequireJavaVersion failed with message:
Error: Detected JDK /usr/lib/jvm/temurin-17-jdk-amd64 is version 17.0.17 which is not in the allowed range [21,).
Error: -> [Help 1]
org.apache.maven.lifecycle.LifecycleExecutionException: Failed to execute goal org.apache.maven.plugins:maven-enforcer-plugin:3.6.2:enforce (enforce-versions) on project checkstyle:
Rule 0: org.apache.maven.enforcer.rules.version.RequireJavaVersion failed with message:
Detected JDK /usr/lib/jvm/temurin-17-jdk-amd64 is version 17.0.17 which is not in the allowed range [21,).
at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:314)
Caused by: org.apache.maven.plugin.MojoExecutionException:
Rule 0: org.apache.maven.enforcer.rules.version.RequireJavaVersion failed with message:
Detected JDK /usr/lib/jvm/temurin-17-jdk-amd64 is version 17.0.17 which is not in the allowed range [21,).
```

I believe this needs updating in order to make the github action work

Open patch-diff-report-tool/pom.xml and compare its Java 17 configuration with the Java 21 requirement shown in the failure log. Update the relevant version setting, then run the GitHub generate-report action to confirm the version-mismatch failure is resolved.

Written by the indexing model from the issue text.