Drop support for Finagle backend

[Taro L. Saito]

While Finagle has been stable and no major change is necessary, its OSS activity has dropped since 2023:
https://github.com/twitter/finagle/graphs/contributors?from=2021-12-12&to=2023-08-04&type=c

Current state:

• No Finagle developers in Twitter
• No release since 2022
• No Scala 3 support
• No dependency updates

I think it's timing to drop airframe-http-finagle and move to Netty backend entirely.

Related: #2440