[Java][FlightRPC] FlightServer can leak during cleanup

### Describe the bug, including details regarding any error messages, version, and platform.

In apache/arrow#38318 , a few Flight tests were flakey due to occasionally leaking during cleanup. The leaks were coming from the server's allocator specifically. A workaround is to cleanup child allocators, however it would be best to determine the root cause of the leak and not have to manually clean-up child allocators.

### Component(s)

FlightRPC, Java

Comece rastreando a limpeza de FlightServer e o ciclo de vida do seu allocator na implementação Java FlightRPC. Revise os testes flaky de Flight descritos em apache/arrow#38318 e reproduza o comportamento da limpeza. O trabalho estará concluído quando o vazamento do allocator do servidor for identificado e corrigido sem limpar manualmente os allocators filhos, e os testes afetados passarem.

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