Support Jetty 12

[d-t-w]

Jetty 10 and 11 are now "Community EOL".

jetty/jetty.project#10485

Suggests it's beyond time to implement Jetty 12.

[d-t-w]

The time has come to move to Jetty 12 (9,10, and 11 are all now EOL).

Jetty 12 Architectural Overview: https://webtide.com/introducing-jetty-12/
Websockets in Jetty 12: https://jetty.org/docs/jetty/12/programming-guide/server/websocket.html#jetty
Migration guide: https://jetty.org/docs/jetty/12/programming-guide/migration/11-to-12.html

Current Clojure state-of-play for Embedded Jetty + Websockets (other than this project):

Ring Jetty9 Adapter

• Limited to simple Embedded Jetty + Websockets
• Uses Jetty 12
• Ring dependency reduced due to choosing Jetty 12 without servlet implementation.

Project: https://github.com/sunng87/ring-jetty9-adapter
Apropos of nothing, this is a cool ticket: sunng87/ring-jetty9-adapter#104
Project owner does not use websockets: sunng87/ring-jetty9-adapter#115 (comment)
Ring WS integration fix: sunng87/ring-jetty9-adapter#121

Ring

• Jetty presented as an 'adapter' to Ring
• Uses Jetty 11
• Plans to advance to 12 in the future

Project: https://github.com/ring-clojure/ring
Ring 'bump' to Jetty 12: ring-clojure/ring#512
Interesting Ring-Cors / Jetty issue to be mindful of: ring-clojure/ring#515

Sente

• Uses Jetty 11, some current issues

Reintroduce Jetty9 support: taoensso/sente#426
Add Jetty11 adapter: taoensso/sente#447
Sente Ring 1.11 adoption (standard protocols): taoensso/sente#443
Sente Rj9a request: sunng87/ring-jetty9-adapter#89
Jetty related 500/upgrade error: taoensso/sente#460

[d-t-w]

Goals for This Ticket

• Update to Jetty 12
• Limit any dependency on Ring (remove if possible)
• Integrate latest Sente, consider the cost of removing/replacing

Technical Decisions

• Use Jetty Handler impl (no servlets)
• Use Jetty Websocket impl (no need to conform to ring ws spec)
• No code-sharing between 12 + 9/10/11 (EOL projects can continue to share, 12 separate)