Make `defaultUseHttp2Preface` flag more explicit about armeria HOT 2 OPEN

I wrote this issue because it would be good for users to freely control connection preface or Upgrade: h2c using the flag for debugging purposes. That said, I have doubts about whether the feature is really necessary. It may bring unnecessary complexity.

We need to hear the opinions of other maintainers and users, but we may treat the existing behavior, which defaultUseHttp2Preface affects h2c, as a bug.

I think applying the defaultUseHttp2Preface flag only to http is straightforward and makes sense.

from armeria.

Hi, I'd like to work on a PR for this, but I'd like to confirm the following point before I start working on this.
Would there be a real or at least a debugging use case of using Upgrade: h2c header over HTTP/1.1 to the endpoints explicitly configured to use h2c?

Asking this because...:

1. If the endpoint doesn't support Upgrade: h2c over HTTP/1.1, Armeria client will throw SessionProtocolNegotiationException.
2. A server that supports Upgrade: h2c upgrade will almost always support HTTP/2 prior knowledge.

from armeria.