DGS-2989 Update function return type due to changes in parent class

[sanjay-awatramani]

Updated return type of onJoinPrepare due to changes in parent class.
Refer below snippets for guidance on the return type
`* @return true If onJoinPrepare async commit succeeded, false otherwise

---

    // return true when
     // 1. future is null, which means no commit request sent, so it is still considered completed
     // 2. offset commit completed
     // 3. offset commit failed with non-retriable exception`

source: https://github.com/apache/kafka/pull/11340/files
Also refer https://github.com/apache/kafka/pull/11340/files#diff-9075bf1ce4e8d878d16f25add001986e3e319cd5ce93290e4f10e4fd60e60155L221 for another example of a usage similar to ours

[rayokota]

Thanks @sanjay-awatramani , LGTM

[guozhangwang]

Not an expert on SR, but just for the API changes itself LGTM.

[dragosvictor]

LGTM provided it passes the build. (Triggered a new one here after merging #2173).