fix(platform-express) respect existing parser middlewares when using Express 5

Express 5 made the router public API again and renamed the field from app._router to app.router.
This broke the detection mechanism whether a middleware named "jsonParser" or "urlencodedParser"
is already registered or not.
Unfortunately, #14574 only fixed the issue partially.
This commit now uses app.router everywhere.
To avoid future regressions a test was added to verify the expected behavior.

Author: luddwichr

packages/platform-express/test/adapters/express-adapter.spec.ts

@@ -5,7 +5,7 @@ import * as sinon from 'sinon';
 
 afterEach(() => sinon.restore());
 
-describe.only('ExpressAdapter', () => {
+describe('ExpressAdapter', () => {
   describe('registerParserMiddleware', () => {
     it('should register the express built-in parsers for json and urlencoded payloads', () => {
       const expressInstance = express();