[UMBRELLA] Track Apache Fluss post-graduation tasks

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found no existing post-graduation umbrella.

### Description

Apache Fluss has graduated from the Apache Incubator to a Top-Level Project (TLP). This umbrella tracks the remaining post-graduation cleanup work. Graduation preparation was tracked separately in #2562.

### Willingness to contribute

- [ ] I'm willing to submit a PR!

No file, test, or entry point is named. Review the graduation preparation tracked in #2562 and identify a specific remaining post-graduation cleanup task; the issue does not define what completion looks like.

Written by the indexing model from the issue text.