collab: Update how mode is displayed in root endpoint (#15911)

This PR adjusts how we display the "mode" collab is running in on the
root endpoint.

It's minor, but it does make things a bit cleaner.

Release Notes:

- N/A

crates/collab/src/lib.rs

@@ -235,7 +235,8 @@ impl Config {
 }
 
 /// The service mode that collab should run in.
-#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+#[derive(Debug, PartialEq, Eq, Clone, Copy, strum::Display)]
+#[strum(serialize_all = "snake_case")]
 pub enum ServiceMode {
     Api,
     Collab,