Update Web FAQ so to mention the current state of service workers

[johnpryan]

Page URL

https://docs.flutter.dev/platform-integration/web/faq

Page source

No response

Describe the problem

We removed the service worker so now the last section of that page may need a slight update (the --pwa-strategy bits).

Expected fix

Any references to outdated flags or features to be removed

Additional context

No response

I would like to fix this problem.

• I will try and fix this problem on docs.flutter.dev.