Tracking Issue for Netty being Experimental.

Despite Netty being the default and recommended transport, we mark is as experimental, implying it is not stable. Once the transport layer is stable, this should be promoted.

Either we should commit to supporting it, or commit to a subset of the methods exposed.

Specific usages:
- `GrpcSslContexts`
- `GrpcSslContexts.configure`
- `NegotiationType`
- `NettyChannelBuilder`
- `NettyServerBuilder`

Start by auditing the listed Netty entry points: GrpcSslContexts, NegotiationType, NettyChannelBuilder, and NettyServerBuilder. Determine the supported API scope and what evidence is needed to promote the transport from experimental; done requires a decided support commitment or subset, followed by the corresponding status change.

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