Extension deprecation: Completely remove frontend routes if feature is disabled

Created by: philipp-spiess

Depends on #39523 Based on #39607

I noticed that some routes would still work even with both feature flag turned on/off accordingly. Specifically:

• /extensions/JohnnyMustaine/1

So I audited all the React routes we have exposed for extensions and made sure they are all off when legacy extensions are disabled.

Test plan

Before

After

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.