Maven checkstyle does not support comma separated list for custom import order

### Affected version

3.6.1-SNAPSHOT

### Bug description

The custom import order rule https://checkstyle.sourceforge.io/checks/imports/customimportorder.html has deprecated the `###` as a delimiter for the order of packages in the custom import order.

A comma separated list can be passed but is not supported by maven checkstyle:
```

```

Fails with
```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-site-plugin:3.20.0:site (default-site) on project MCHECKSTYLE-357: Error generating maven-checkstyle-plugin:3.6.1-SNAPSHOT:checkstyle report: Failed during checkstyle configuration: cannot initialize module TreeWalker - cannot initialize module CustomImportOrder - Cannot set property 'customImportOrderRules' to 'STATIC, STANDARD_JAVA_PACKAGE, THIRD_PARTY_PACKAGE': InvocationTargetException: Unexpected rule: STATIC, STANDARD_JAVA_PACKAGE, THIRD_PARTY_PACKAGE -> [Help 1]
```

For a minimal example you can edit `src/it/MCHECKSTYLE-357/pom.xml` to use the new syntax and then run it with
`mvn invoker:run -Dinvoker.test=MCHECKSTYLE-357` to see the above error.

Start with the minimal example in src/it/MCHECKSTYLE-357/pom.xml and run mvn invoker:run -Dinvoker.test=MCHECKSTYLE-357 to reproduce the CustomImportOrder failure. Read the plugin code involved in processing customImportOrderRules, then verify that the comma-separated configuration runs successfully through the integration test.

Written by the indexing model from the issue text.