Server: Use min instead of max to limit the number of threads (#17421)

## Summary

Prevent overcommit by using max 4 threads as intended.

Unintuitively, `.max()` returns the maximum value of `self` and the
argument (not limiting to the argument). To limit the value to 4, one
needs to use `.min()`.

https://doc.rust-lang.org/std/cmp/trait.Ord.html#method.max

crates/red_knot_server/src/lib.rs

@@ -30,7 +30,7 @@ pub fn run_server() -> anyhow::Result<()> {
     // by default, we set the number of worker threads to `num_cpus`, with a maximum of 4.
     let worker_threads = std::thread::available_parallelism()
         .unwrap_or(four)
-        .max(four);
+        .min(four);
 
     Server::new(worker_threads)
         .context("Failed to start server")?