[ui] Rename nav/settings flag (#21335)

## Summary & Motivation

By request from @braunjj.

## How I Tested These Changes

View user settings dialog.

js_modules/dagster-ui/packages/ui-core/src/app/getVisibleFeatureFlagRows.tsx

@@ -25,7 +25,7 @@ export const getVisibleFeatureFlagRows = () => [
     flagType: FeatureFlag.flagUseNewOverviewPage,
   },
   {
-    key: 'Use new settings page',
+    key: 'New navigation (experimental)',
     flagType: FeatureFlag.flagSettingsPage,
   },
 ];