title_bar: Put plan behind new-billing feature flag (#29540)

This PR puts the displaying of the plan in the user menu behind the
`new-billing` feature flag instead of the old `zed-pro` feature flag.

Release Notes:

- N/A

crates/feature_flags/src/feature_flags.rs

@@ -59,6 +59,16 @@ impl FeatureFlag for Assistant2FeatureFlag {
     const NAME: &'static str = "assistant2";
 }
 
+pub struct NewBillingFeatureFlag;
+
+impl FeatureFlag for NewBillingFeatureFlag {
+    const NAME: &'static str = "new-billing";
+
+    fn enabled_for_staff() -> bool {
+        false
+    }
+}
+
 pub struct PredictEditsRateCompletionsFeatureFlag;
 impl FeatureFlag for PredictEditsRateCompletionsFeatureFlag {
     const NAME: &'static str = "predict-edits-rate-completions";