[docs] Upgrade flink docker image version to latest Flink 2.2 for quickstarts

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Description

Currently, the quickstart demos use Flink 1.20 as the base image version. However, many new features—such as Delta Join—are only available in newer versions like Flink 2.2. Upgrading the base image to Flink 2.2 would allow users to reuse the existing Docker Compose environment to easily try out these advanced features.

### Willingness to contribute

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