Upgrade Flink dependency to 1.20.3 and 2.1.1

[wuchong]

Search before asking

• I searched in the issues and found nothing similar.

Motivation

Flink 1.19.3 and 1.20.3 and 2.1.1 has been released:
https://flink.apache.org/2025/11/10/apache-flink-2.1.1-release-announcement/
https://flink.apache.org/2025/10/09/apache-flink-1.20.3-release-announcement/
https://flink.apache.org/2025/07/10/apache-flink-1.19.3-release-announcement/

We should upgrade the dependency version.

Solution

No response

Anything else?

No response

Willingness to contribute

• I'm willing to submit a PR!

[gyang94]

@wuchong quickly made a PR for it. Built successful locally.

[qining-mj]

@wuchong Can I work on this issue?

[chucheng92]

I recently read the fluss documentation and also encountered this issue. Note that it's not just a dependency issue; some titles and links in the docs pointing to older releases like 1.20.1 are no longer valid (e.g. 1.20.1-bin.tgz). We need to update the docs as well.