Switch to ring/ring-jetty-adapter

[jacobobryant]

Now that the Ring spec + official Ring Jetty adapter supports websockets, we could switch to that instead of using info.sunng/ring-jetty9-adapter. The version of that lib that Biff uses--0.17.2--uses a non-Ring-spec API for websockets (since the Ring websocket spec didn't exist at the time it was created). So switching to ring-jetty-adapter would be a breaking change unless we can have Biff's use-jetty component start translating the old ring-jetty9-adapter websocket response format into the Ring spec format. Also, Biff users may have code that requires ring.adapter.jetty9 directly; e.g. the starter app uses ring.adapter.jetty9/send! for sending websocket messages. Though maybe that function would actually still work even if we switched to ring-adapter-jetty?

If we didn't want to deal with breaking changes, we could upgrade to a 0.22.x version of ring-jetty9-adapter at least, which has the same websocket API. I see that 0.17.x is no longer maintained (version matrix). However I think it'd be better to just go ahead and switch to the Ring spec. Even the latest version of ring-jetty9-adapter has switched its websocket format to that. It'll be easier for users if they aren't surprised by the non-standard websocket API.

Looks like ring-jetty-adapter still supports Java 11, so no breaking changes there.

Should we just leave ring-jetty9-adapter in Biff's deps for a while? If that works, there'd be zero breaking changes. We could have use-jetty keep using ring-jetty9-adapter, at least by default. Either provide a different Biff component that uses ring-jetty-adapter, or add a config option that tells use-jetty which one to use. In this case, we would upgrade ring-jetty9-adapter to a 0.22.x release, which--like ring-jetty-adapter--depends on Jetty 11.x.

So the todo list here:

• Add ring-jetty-adapter to Biff's dependencies.
• Update the starter app.
• See how much backwards compatibility we can provide--maybe we can get away without any breaking changes. If we don't go that route, remove the ring-jetty9-adapter dependency.
• Update the tutorial.
• Update any other docs that include websocket code (e.g. check the reference docs).
• Draft a release highlighting the breaking change(s), if there are any.