### _Why are the changes needed?_
Update docs to mention that Kyuubi requires Flink 1.15+ now
### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible
- [ ] Add screenshots for manual tests if appropriate
- [ ] [Run test](https://kyuubi.readthedocs.io/en/master/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#4721 from pan3793/doc.
Closes#4387
4fdd9082b [Cheng Pan] [KYUUBI #4387][DOCS][FOLLOUP] Update Flink version requirements
Authored-by: Cheng Pan <chengpan@apache.org>
Signed-off-by: Cheng Pan <chengpan@apache.org>