Remove uvicorn concurrency limit causing 503s

The limit_concurrency=10 was causing widespread 503 Service Unavailable
errors during the AMD hackathon. SSE connections from CLI submissions
hold a slot for the entire GitHub Actions run (10+ minutes), so a
handful of concurrent users would exhaust all 10 slots and block
everything else including web UI requests.

Author: Mark Saroufim

src/kernelbot/main.py

@@ -39,7 +39,7 @@ def create_uvicorn_server() -> uvicorn.Server:
         host="0.0.0.0",
         port=int(os.environ.get("PORT") or 8000),
         log_level="info",
-        limit_concurrency=10,
+        limit_concurrency=None,
     )
     return uvicorn.Server(config)