Mantis #30823: add Course AboStatus to XML export & import

[GitHamo]

This PR addresses the issue outlined in Mantis ticket #30823, where XML course export is missing the <AboStatus> element, and the XML import ignores it.

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⚠️ PR is for release_9, and due to differing XML schema validations across versions, separate pull requests are necessary for release_10, release_11, and trunk.

Please advise on how to proceed with these PRs.

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Key changes:

1. class.ilCourseXMLWriter.php: updated to include the <AboStatus> element in the exported XML file. It retrieves the subscription status from the course object and writes it into the XML.
2. class.ilCourseXMLParser.php: updated to handle the new <AboStatus> element. During import, it reads the value from the XML and sets the subscription status on the newly created course object.
3. ilias_crs_9_0.xsd: updated to include the AboStatus element. This makes the element a valid and required part of the course XML structure for ILIAS 9, ensuring that exports are compliant with the schema. (If not done for each version, any course import with <AboStatus> would fail)

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Test Steps:

1. Export an existing course.
2. Open the generated export file (e.g., Modules/Course/set_1/export.xml) and verify that the element is present within the course data.
3. Modify the value in the XML and import the course to confirm correct import behavior.