### What changes were proposed in this pull request?
Remove out-of-dated flink 1.14 and 1.15.
For more information, please see the discussion thread: https://lists.apache.org/thread/njho00zmkjx5qspcrbrkogy8s4zzmwv9
### Why are the changes needed?
Reduce maintenance burden.
### Does this PR introduce _any_ user-facing change?
Yes.
### How was this patch tested?
Changes can be covered by existing tests.
Closes#3029 from codenohup/remove-flink14and15.
Authored-by: codenohup <huangxu.walker@gmail.com>
Signed-off-by: Cheng Pan <chengpan@apache.org>
a57238024e