Proposal: Migrate the JDK 21 annotations typetools/jdk to JDK 25

This issue proposes migrating the existing annotated JDK 21 repository (in typetools) to the OpenJDK 25 release.

According to the New Contributor Projects - Case Study: JDK 25 documentation, merging existing JDK 21 annotations into JDK 25 is the principal first step for this project. I have reviewed the checker-framework and typetools repositories and have not found any active work or open PRs regarding this specific migration.

I am highly interested in leading this migration. I have already initialized the merge process locally and have successfully resolved approximately 40% of the conflicts.

I opened this ticket because the following reasons:

1. Verify that this migration is currently necessary and valuable for the Checker Framework.

2. Ensure that no other contributors are currently working on this task to avoid duplicated effort.

3. If the above are confirmed, I would like to share my detailed merge strategy and feasibility analysis (based on the project README) for review and feedback.

Start with the existing annotated JDK 21 repository in typetools/jdk, the JDK 25 case-study documentation, and the project README. Review the local merge work and remaining conflicts, then confirm that the annotations are migrated to OpenJDK 25 and that the resulting repository integrates with the Checker Framework.