build: default the dev app to M3 (#29871)

Sets the dev app to M3 when landing on it for the first time.

Author: crisbeto

src/dev-app/dev-app/dev-app-state.ts

@@ -46,7 +46,7 @@ export function getAppState(): DevAppState {
       systemTheme: false,
       rippleDisabled: false,
       strongFocusEnabled: false,
-      m3Enabled: false,
+      m3Enabled: true,
       direction: 'ltr',
       colorApiBackCompat: true,
     };