### What changes were proposed in this pull request?
Support Flink 2.1.
### Why are the changes needed?
Flink 2.1 has already released, which release notes refer to [Release notes - Flink 2.1](https://nightlies.apache.org/flink/flink-docs-master/release-notes/flink-2.1).
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
CI.
Closes#3404 from SteNicholas/CELEBORN-2093.
Authored-by: SteNicholas <programgeek@163.com>
Signed-off-by: SteNicholas <programgeek@163.com>