Filter nowarn annotation from jar

[rossabaker]

Alternative to #4067. We just simply filter the @nowarn annotation out of the jar.

Pros:

• No new core dependencies
• Preserves fatal warnings without further complexity

Cons:

• Compat trait will need to be copied to a scala-3 directory on the Dotty branch.
• Doesn't steer our source toward modernity.
• MiMa doesn't catch that the annotations are missing. But we'll note it and move on, and it's 2.12 only.

[rossabaker]

I'm leaning toward this being the better approach, and conservatively putting it on main instead of 0.21.