Investigate usage of deprecated ApplicationProtocolNegotiator

In #3390 we ignored the deprecation warning. We need to investigate whether we need the checks (and so tweak netty to keep the API semi-public) or whether we want to just delete them.

Start by reviewing the deprecated ApplicationProtocolNegotiator usage and the context from issue #3390. Determine whether the checks still serve a purpose and whether Netty must retain the API as semi-public. Done means documenting the decision and either preserving the required API or removing the checks.

Scritto dal modello di indicizzazione a partire dal testo della issue.